Since you are buying tickets as part of a WDW Resort Package and then upgrading them,
there is a way to do that properly and compute the exact price.
USE the package tickets at least one time to go into a theme park BEFORE you do the upgrade.
Then-
To see the price to do the upgrade, check the NEW FULL GATE PRICE of the ticket that you HAVE.
(Example, if you got a 6-day ticket plus a "free day,"
you would check the gate price of a brand new 7-day MYW ticket.)
Then, subtract that price from the NEW FULL GATE PRICE of the ticket that you WANT. (Example, 10-day MYW ticket with Non-Expiring option.)
Ticket prices are listed
HERE.
When you compute the difference in the cost of the two tickets, add 6.5% FL tax and that's the full cost of the upgrade.
Yes, you will get the full NEW gate price for the ticket that you already HAVE even after you have
used at least one day of that ticket to enter a theme park.
The act of using the original ticket to go into a park will "lock" that ticket's value
at the new gate price for the purpose of the upgrade transaction.