I've only been playing for about a month and there's definitely been a bit of movement on the server I play on. For the most part the change is positive, even in inactive cities, there's a greater gold boost. Prior to the move, there were very little neighbors, now there's not a single gold mine and I've completed a 5 providence away radius. The best positive by far with cities moving is substantially more trade activity.
Still, there are some cities which are permanently inaccessible. There's a few who were in the middle of a main hall upgrade and I suppose never logged back in again so there are no help options or gold boosts there.
I have noticed that some players are gone entirely over the past few weeks. These were all cities that had never made it past the original tutorial, so I wonder if there's account deletion after a certain period of inactivity.
All of my boosts have stayed the same.
I find that the outer edge changed a little. To the south has always been pretty sparse, but if I complete the providences in the north first, by the time I get to the south, there's more cities there. Not sure if this is because of accounts signing up or cities moving.
I've kept all of my active neighbors.