What, if any, version tags should be applied to a Q&A that has answers relevant to a range of versions?
This is another situation that supports the original version tag guidelines as used on other exchanges - don't use one unless the question or issue is specifically related to the version. That said, this question doesn't appear to be considering initial tagging, but rather editing, modification, and merging after the fact. In such a case, and with the horses already out of the barn...
If you're already making an edit and you know it's not a version specific question (ie timeless), particularly if your changes are related to cleaning up duplicate questions from differing versions, remove the version tags.
If not consolidating questions of differing versions (but still doing an edit - tag edits just to add or remove a version tag should be avoided in my opinion, again because of trivial edits and differing philosophies on version tag use), I would probably leave version tags alone because of the current situation with them (being that they are frequently added to all new questions).
I do not support adding either the highest applicable version tag or especially a pair of tags that are supposed to indicate the range of version applicable. Slots available or not, that seems like it goes against a couple of different tagging system principals and further dilutes the value of version tags (from either viewpoint).
At one time I was an advocate for version tags but over the years my view on them has changed to being that they are of limited use.
Nowadays, as long as the version(s) being asked about (and presumably used) is/are mentioned in the body of the question, then I think they are best not tagged.
Likewise, if answers cover a range of versions and are suspected not to apply to all, then any answer which specifies the version(s) that it does apply to is more likely to receive an upvote from me.