Ticket upgrading/dollar credits always seemed complicated to me, but it's actually pretty easy.
Think of it this way: UPGRADING means you are given full credit for what was paid for the ticket you're trying to upgrade.
From that base you can sort of work your way down.
1.) You can only get full credit back for unused admission; EXCEPT: You can upgrade used admission within 7 days of the first use of the admission. When upgrading to an annual pass, if you've used any plus options on your pass, you can only upgrade to a premium annual pass.
2.) Notice how I said you're refunded based on what WAS paid? Well, you are given credit based on the amount that DISNEY charged for the ticket. For example - discount ticket brokers buy tickets from Disney in bulk, and get a cheap rate. They then mark the tickets up. You would get credit for what your BROKER paid to DISNEY. Not what YOU paid to your BROKER. Make sense?
That should cover bases for the OPs question, but just cause I'm bored....
If you have USED admission, and would like to use it as credit for a new piece of admission media, you then would ask for dollar credits. (Don't ask for an upgrade! They'll tell you it can't be done!

)
Disney will give you a prorated credit based on 2 things:
a.) How much Disney charged for the ticket (see #2 above)
b.) How many days are remaining
If you have any sort of plus options, they don't have any value in dollar credits.
So let's say you have a 6 day park hopper plus. Let's say you bought it direct from Disney for $240. Let's say you've used 2 days.
$240/6 = $40
$40 X 4 = $160
You would get $160 credit. If you used all three plusses, you'd get $160. If you used NONE of your plusses, you'd get $160.
I think I covered everything!
