When somebody changes their name I think it would be valuable for a period of time to simply mention their previous name in a tooltip while hovering over their new username in comments and answers. This avoids confusing people while reading comments with references that may be outdated, or somebody sees a name they do not recognize.
A simple "Previously, oldname" would work well to reduce confusion while a user is hovering over somebody's username. It wouldn't have to be a permanent fixture, but could be active for, say, 30 days after the fact.