OK let say I am set to go to the Prep 21d - 80d but I end up winning my present leagues championship will i then get forced into the University 61d - 120d???

Age caps don't care if you win(except for placement within the age cap). You will move to 61-120 because you currently have 3 players over 100 days old and another who is at 79 days old today so he would also push you up if you keep him. If you cut those guys at the end of the season you would go to prep. if you keep any player who ends the season 81 days old you go up to university.

already set up to release them at seasons end.

You will probably go to an elite tier, but still in 21-80 age cap league.