When you upgrade tickets to an AP, they will calculate the per day cost according to what you paid for your pass. Then they will credit you for those unused days toward an AP, which will be effective the date of this transaction.
The MYW tickets actually have a value that decreases according to how many days you purchase. I believe a 10 day MYW pass works out to only around $29 for a per day admission. This makes the break even point around 14 days versus the old 8 day rule of thumb for a better value with an AP.
Definitely check with Guest Services for the most accurate information for your particular case. Good luck!!