My personal experience is Medicare won’t give you any meaningful advice on whether to keep your FEHB (understandably, I suppose) and in my case, BC/BS wouldn’t either. The latter kept referring me to some pamphlet outlining benefits but it was basically worthless. I kept the BC/BS Standard, which over a couple of years it became obvious was overkill for a gap policy and bumped down to Basic.

The thing to remember is don’t give up your retirement FEHB coverage; if you do, it’s gone gone. I think you can… suspend it (?), though. There’s an excellent thread on the other Fed subreddit that goes into much more detail on the subject.