if you have 40k left over in d3 you should be finding better players.
think of it this way, right now you have 40k, youll lose 30 and keep 10
if you spend 10 k upgrade at a position, youll have 30k. youll lose 23.5 and keep 7.5, so you have a much better player (or two) and it costs you $2500.
if you spend 20k to upgrade players, youll have 20k left. youll lose 15 and keep 5.
why would you want to keep it and lose it?
anyways, to answer your question...the only reason you lose carryover is if you do not fill all of your scholarships. if you have 15 open scholarships and sign 15 guys, regardless of position, you get to keep it.