Actually, that's not *quite* true... If you have the Water Parks & More option and you've USED any of those WP&M admissions, then you're forced to upgrade to a Premium AP. But before you use them (or before you use the ticket at all), they're just considered a dollar-amount value on the ticket.
As for the answer to the OP, you're out of luck, unfortunately. Disney does not give refunds when trading *down* tickets. You're either stuck using the 10-day ticket as-is, taking the hit in the wallet and end up paying *more* for an AP than you normally would, or upgrading to a Premium AP.
When you're buying a ticket through a reseller, the trick is to buy one that gives a good discount, but make sure that its gate value is still LESS than the ticdket you want to upgrade to.
EDIT: Also, as long as you use a major theme-park admission before you upgrade the ticket (to lock-in the gate-price value), even though you only paid $555 for it, that ticket's value when applying it toward an upgrade is the current gate-price for it, $592.14. So the upgrade to a Premium AP would be about $67.
-Rob