If you get a freepass to disneyland on your birthday, and you want to get a park hopper. Could you upgrade it to a park hopper and pay the difference in price between a single day ticket and a park hopper ticket?
Yes! The FOYB ticket carries a cash value of $69 (adults) or $59 (children), and can be put towards a park hopper. The result is that you pay the remaining $25 and you get a park hopper! This cash value can also be applied towards any other upgrade, including turning the ticket into a 2Fer for free, upgrading it to an AP (including payment plans) or turning it into any multiple day hopper by paying the difference.
"Remember 'Old Yeller'? We shot the dog." - Roy E. Disney