Kim Robins medal 5000 9 20
if changing from one league to another do i lose the ability to use my pro level drivers and automatically drop back to rookie upon joining a new one?
Jack Basford medal 5170 
9 20
No, you stay in the Pro tier and can move to the Pro tier in any other league. You get to keep your entire team.
There's a dialog which warns you of all consequences when changing leagues, and anything it doesn't say on the list won't happen. :)
