Unless I'm mistaken, they're paid the same regardless as long as they're on the 40man roster.
Players in the minors who are NOT on the 40 man roster are paid a minor league salary and if called up (rather unusual) are of course promoted to the 40 man roster and would begin receiving the ML minimum salary pro-rated to the amount of time they're on said 40man roster.
Also, (and I'm happy to be corrected) I'm pretty sure that if a player is dropped from the 40man roster, he has to clear major league waivers before he can go to work for the (presumably) AAA club.
Such players are those you hear "designated for assignment". These are the guys who are pretty much done with chances at the team's ML roster and they'll either retire or perhaps resign with another ML club if possible.