There are of course very good reasons for not allowing a Scale to be altered once it is in use. I'm sure anyone who thinks this through will understand why, and the potential impacts on learners of changing a Scale after it has been used!
I do like the idea of archiving, or perhaps "Locking" a Scale.
I use the word locking because it's consistent with the concept of locking a forum discussion.
And yes, locking a Scale would be really good, if it could then not be used for new activities.
Given this, it would makes sense to have a "Replicate" Scale option, to make a quick copy, before locking the old one.