I've been cleaning up some tag errors and have discovered that there is no easy way to retag a closed question. There should be. Can this be added?
In particular, I was trying to retag "opensource" to "open-source"
I'm assuming you mean this one? Locked questions (such as due to migration) cannot be edited, including tags. But they are not immune to global retags by ♦ moderators, and that also has the advantage of not spamming the front page.
Since there were 23 opensource and >1300 open-source, I'll merge into open-source for you:
I've only noticed not being able to retag closed items when they've been migrated. Migrated items are locked, meaning they cannot be edited in any way.
Retagging, even of closed items, is not rep-based. On SU, I have ~1200 rep (not enough to edit, but enough to retag). I was able to retag this question successfully.
So the question is: Were you trying to retag a question which was closed due to migration? (You'll see a message at the bottom of the question indicating such.)