Doesn’t need to be a crime. The grade determination is “at what rank he admirably served. Col (now 1Lt Ret, USAF) Mike Murphy can tell you all about it.
That would be honorably, not admirably. Oy…
The allegations, if true, might well have cost him his star, and seen him retired as an O6, if they came to light while he was retired. But taking his star after he retired? I expect memoirs to be a lot less candid if this is a thing for retired service members who haven’t committed the crime of being on good terms with Donald Trump.