The general policy, as a previous poster found out, is that you pay the difference between what you originally paid for the ticket you are holding and what the ticket you now want costs when you make the upgrade.
Disney's system knows what was originally paid for the ticket. That's the important number in this situation, not what it's 'worth'..... A similar example is this - if you buy a ticket from a discounter, Disney will see how much the discounter paid Disney for the ticket and *that* amount is what you'd get 'credit' for, not what you paid the discounter or how much it would cost to buy a similar ticket today.
I"m not 100% sure I was reading the other post correctly - it sounded like they had to pay for the options that were originally free when they upgraded the ticket. That may or may not affect whether you upgrade that particular ticket. It'll probably pay to crunch a few numbers on your various options.