Seems the easiest solution to that would be to simply have a comparison view. The permissions are setup using some sort of standardized method, yes? Config file, GUI (which just alters config file), whatever. Certainly it wouldn’t be hard to simply grab and organize a list of perms from both instances and toss em up side by side. Could even add notes (i.e. if photo storage on Instance A > Instance B notify user that “migrating to this instance may cause some larger photos to be removed from your account”).

I’m not a professional programmer (sys admin, so mostly just automation) but there are certainly solutions to this.