I was glad when I could change a username from testacct1 
Banging an existing inconsistent username to Discourse’s defination might have high effect during migration i.e. old names in to pseudo names
By this I mean “not real but as similar as possible”, a fake. (so maybe faux-code, meh)
IMHO making changes to have a looser definition is not a wise decision.
giving affected members due time and sufficient Notice is a must do.