The latest POWO refresh now follows FNA and VASCAN. The name S. densa var. scopulorum is now S. scopulorum, and the var. name should be removed and the species name swapped into its place. Thank you!
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:232290-2
http://dev.floranorthamerica.org/Selaginella_scopulorum
https://data.canadensys.net/vascan/name/Selaginella%20scopulorum

chiming in from your comment here

I understand you want to follow POWO by elevating Selaginella densa scopulorum to Selaginella scopulorum
to do this you'll need this draft swap
https://www.inaturalist.org/taxon_changes/142672
but if you click "Analyze Unintended Disagreements" since there's 82 unintended disagreements, you should also split
https://www.inaturalist.org/taxa/136990-Selaginella-densa
more info on how to do that here https://help.inaturalist.org/en/support/solutions/articles/151000015337-section-d-how-to-respond-to-a-flag-requesting-to-split-a-taxon

Here's what I have done:

-- Swapped the varieties over to new homes -- Benefit: Narrows down the input ids for the upcoming split, and prevents new observations from being submitted under the old name.
-- Created a split draft -- https://www.inaturalist.org/taxon_changes/142828
-- Created atlases for the three species according to POWO maps.

Big con: The outcome of ids under the current atlases in the split; 71% of ids will be bumped to genus level due to overlapping distributions. Refining may be needed.

-- 2886 input ids

-- 57 --> S. scopulorum
-- 753 --> S. densa
-- 0 --> S. standleyi
-- 5 --> Selaginella due to being submitted outside known distribution
-- 2071 --> Selaginella due to overlapping distributions

Lastly:

-- Unintended disagreements for standley -- https://www.inaturalist.org/observations?id=139821217,166535462,173288406,178318621,182992474,183514020,183847662&place_id=any&verifiable=any
-- Unintended disagreements for scopulorum -- https://www.inaturalist.org/observations?id=164897796,170139243,172026312,178931427,180699798,181854524,183005289,183237970,183268560,183846943,184289576,184334883,184637269,185082123,185333632,185518616,185938176,186224713,186424389,187010792,187205021,187506070,187936028,205602825,207597004,60467198&place_id=any&verifiable=any

Once the split is completed and committed, we'll swap nominate densa into the species to make it monotypic.

PS the reason I have set out to put in a var. for every S. densa s.l. observation is that it would preserve the name S. densa at the end of the swap process as described by Matthias, and avoid having all those many many correct S. densa observations go back to genus.
I decided to start on the US coast in areas where S. densa does not occur (according to the FNA Legacy map polygons and local floras), so I haven't yet gotten to reviewing the US states farther east where any of the three taxa can occur, that is where I'll be adding in var. densa to many of the S. densa s.l. observations.
Then I'll do Canada.

@margaret_eaglecap Here's my step by step process:

1 - Swap the varieties into species taxa, except S. densa var. densa -- This will result in all variety ids being transferred to their species (ex. all S. densa scopulorum ids --> S. scopulorum). However, if an observation has a S. densa id and a S. [densa] scopulorum id, it will cause an unintentional disagreement.

2 - Due to high unintentional disagreement rate: create split of S. densa -- Splitting S. densa is a necessity. For a split to be done efficiently, atlases have to be made.

3 - Creating atlas -- The atlas is a map of which each species occurs according to geopolitical borders. The main issue is, there appears to be a lot of sympatries between the three species. Splitting with the atlases I've created (according to the maps on POWO) would result in the following. Keep in mind that the split will only change S. densa s.l. ids.

-- Alaska --> S. standleyi
-- Washington, Oregon, California, Nevada, Utah, Texas --> S. scopulorum
-- Saskatchewan, Manitoba, Quebec --> S. densa
-- British Columbia, Alberta, Idaho, Montana, Wyoming, Colorado --> Selaginella

4 - Once split is committed, swap S. densa densa into S.densa

Regardless of how this goes down, if there is an area sympatry, those ids will go back to genus. It means all those observations will have to be reidentified in the first place, and this can be done pre- or post-split.

@marget_eaglecap Yes, if you do pre-split ids, if any are true S. densa, it has to be identified as S. densa densa to avoid the id being bumped to genus.

@matthias22 I understand your thought process, but how does that method differ from splitting? The end results are going to be the same, a majority of densa will be bumped to genus and will have to reidentified. Does it really matter there may be unintentionally disagreements? It just seems like your method is just adding an extra step (a new taxon will have to be made for densa) that really doesn't seem all that necessary.

@matthias22 but there's regions that only have one species. Doing it your way would make observations in Saskatchewan (202 obs total) genus level ids, even though S. densa (s.s) is the only species in that province. Wouldn't it be better to ensure that all Oregon sightings are swapped to S. scopulorum instead of genus, and avoid reidentification? Splitting with atlases is absolutely necessary to make sure that doesn't happen, but the end result is the same as your method, but with fewer genus level obs.

For the future: please do not make any changes to the taxonomic framework without consulting with the people who are preparing them. I would have committed these taxon swaps myself had they been ready. The purpose of staging swaps is to invite discussion. Committing them is the easy part - it only takes one click of the mouse. Preparing them takes time. I have been discussing this with Margaret for a couple of days so it would come out the way we wanted it to be. Now we are faced with a problem that I am unable to resolve.

What we need is a way to replace densa (s.l.) with Selaginella without causing disagreements (the green vs. red button scenario when you manually add an ID). We need the green button. Perhaps you can figure out a way how to do that. That would save the day. I don't want to discuss taxon splits any further because they are beside the point. We never intended to split densa.

I don't understand what you mean by "no more disagreement" in your first scenario:
Swap + split: Observation in overlap region
Id 1 = S. densa
Id 2 = S. densa scopulorum
Id 2 (swap) = S. scopulorum (unintentional disagreement)
Id 1 (split) = Selaginella (no more disagreement)

In this scenario, ID 1 disagrees with ID2 after the swap and split, which means the observation ID will go back to genus level. That's what we wanted to avoid. IDs 1 and 2 did not disagree before the changes and they should not disagree after.

@matthias22 it sounds like what you are describing is pretty much a split in terms of the inat technical implementation. Anything that is in the overlap of two atlases gets knocked back to genus in a split, and things that are not in the overlap get moved. We can also make the atlases finer resolution, if that helps reduce the areas of overlap.

On the other hand, because so far it seems like the vast majority of observations are Selaginella densa s.s, it may be unproductive to do a swap back to the genus at all and may be better to just solicit help from any active IDers interested in the split. I know the system advice says to do a swap for more than 10 unintentional disagreements, but in my opinion that threshold is borderline absurd, because many IDers can fix 10 IDs in 30-45 seconds (depending on the complexity), whereas fixing 2000 IDs unnecessarily broadened to genus would take anyone at least hours if not days. Any IDers/observers tagged below have thoughts on whether a swap to genus is even necessary?

Also, the FNA does describe the varieties as a complex. Therefore, we can create a new "Selaginella densa complex" for Selaginella densa s.l. if it is useful, and it sounds like it might be. That way, the 1000 or so IDs of Selaginella densa would only get knocked back to complex, instead of being lost in the pool of over 10000 North American needs-ID Selaginella observations. That avoids unnecessarily undoing any existing IDer effort. Does that sound useful to people?

Additionally, it appears the majority of affected IDs were made by just one person, @smmckee. In such situations I think it should be considered common courtesy to get their input on a flag before committing anything related to a split, or at least to give them fair warning. @sbbrown, @eric_lamb, @lysandra, @sedgequeen, @johndreynolds, @bstarzomski, @fmcghee, @picklejar, and @dougbrown also each have 40+ IDs or observations affected.

Do any of you have any thoughts on what should be done here?

@margaret_eaglecap I know that you weren't very fond of creating a densa-complex because it might not be (or isn't?) a monophyletic unit, but I think this would be the best course of action. My personal opinion is that species complexes can be groups of convenience that facilitate getting things identified to recognizable units. It is better to tolerate a few paraphyletic species complexes than having large numbers of observations get stuck at higher levels even though they can clearly be narrowed down to a few species. It is different for higher taxonomic levels (subgenera, genera, etc.) IMO. Yes, those should be natural groups.

@birdwhisperer The FNA key, which is the source for the split (Vascans just says it is following the FNA), and the only reliable secondary source that discusses the issue, unambiguously says "Selaginella standleyi is a member of the S. densa complex" and "Selaginella scopulorum is a member of the S. densa complex".

I don't know if you are aware of some kind of unpublished work or something that questions the monophyly of this complex, but we are supposed to prefer secondary sources and the only relevant secondary source supports the existence of this complex. The primary conditions for complexes on inat are that the existence of the complex be citable to a source outside of inat, and that the complex be useful for inat purposes; we don't have to anticipate all possible developments in the future phylogenetic literature. If anything, if you do anticipate that a major literature revision involving the complex is in the pipeline, that would be a good justification for not doing any disruptive swap at all, because unnecessarily knocking IDs back to genus is destructive to the dataset.