1. If you buy your tickets as park of a WDW Resort Pkg. or if you buy them from a discounter (
undercover tourist, etc.)
then, USE the tickets at least one time to go into a theme park BEFORE you do the upgrade.
2. Once you use the tickets at least once, they can be upgraded at a price that is the same
as the current gate price for a ticket of the same kind.
To upgrade:
Subtract the full new current gate price of the ticket that you HAVE from the full new current gate price of the ticket that you WANT (AP or otherwise...) then, pay that difference price for the upgrade.
3. No difference. Works the same way as above.
BTW, there is no DVC "discount" AP.
There is a separate DVC AP that is sold at a reduced cost to DVC owners.