I suppose it could be changed as long as it's not different from that user's wiki username. If a user on the wiki changes their username, the username on the forum may be changed to match. If the user doesn't have a wiki account, the same 2 per year rule could be applied to the forum for...
If you want to adjust to as little as possible, you can check "Use sidebar menus instead of dropdown menus when possible" in Profile -> Look and Layout to make things look a lot more like they used to.
Here is my plan--people can freely change their display name, but their username stays the same. I just made the username the header of profile pages so you can see who it really is.