How will this work with existing saves? Will they still work or do we at some point (hopefully sooner than later) have to restart?
It won't affect existing saves at all. The name that existed
at the time of the save will remain unchanged. If you only want save games that feature the changed name, then yes, you'd have to replay from the start and resave with that name.
The main reason I pulled the feature was the inability to display changed names during replays. Because of the way that Ren'Py does it, everything is initialized to starting values before a replay is shown, including character names. Most games solve that by asking, point blank, for the player to provide what those names are supposed to be and did
this event happen and what happened when
that decision was made...that's always been too immersion-breaking for my liking. Especially if more than one character name is involved. I requested a change to Ren'Py to allow me to do it properly, and PyTom put a fix in the next Ren'Py release, v.8.5. It should have been released by the time the next UT release is ready to go.