Username changes are rather significant, but I can approve this one.

I mostly put the rule in to prevent people from changing names frequently and to avoid confusion when popular or frequent posters changed their names to something radically different. That would happen on other sites when new fads and memes would come and go, and I even lampooned it during an April Fools' Day joke by changing all the staff names to Friendship is Magic characters.

Typically, I ask those who change their name to put their old name in their signature (e.g., "Formerly known as Felly").