How to properly update Google Play Services?

Question

Google

When I published my app for the first time, I only had a player rating in game services, but no achievements, and when I tried to publish the application, I was told that after publishing it, I would no longer be able to publish game services (?) , so I just put 5 hidden placeholders in the achievements. It was also said that I would no longer be able to delete these achievements once published (?). All this is very strange. Now I'm preparing an update where I want to implement already normal achievements, if possible, remove these stubs. There is also a score of players, which, if possible, it is desirable to save. Game services have the ability to unpublish, but it is not clear what will happen (i.e. whether I can then return game services to the game or not, etc.). It's hard for a beginner to figure it all out.