I've heard that changes, they is a coming.
Inspired by the diarist formerly known as Big Tex, I have a little suggestion:
Once the switch is thrown, give users who wish to do so a brief window to swap out their tired old user name for a shiny new one.
It only seems fair; Times change, websites change, people change, why not user names?
How about a one time amnesty for worn out user names?
Me, I'm still a vegan, but I never really was a militant. And, while I'm not expecting to do so, there is a chance that someday I may find myself living off the land, raising a chicken egg or tasting a brook trout. Stranger things have happened.
Given the chance, I think I would name myself after a mountain, or two.
I may do so anyway. It's not like anybody is going to give a hey about my glorious contributions to the site or stunning enrollment number.