See Post #6 on the thread on which we are now posting.
Post #6 is titled,
"Cost to Upgrade Tickets"
Pertinent sections reproduced below-
------------------------------
Ticket bought from an Authorized Reseller/Discounter:
-or-
Ticket bought as part of a WDW Resort Package:
-or-
Ticket bought at a Disney Store:
A __ Created BEFORE Feb.12,2017 - (2016) Prices HERE.
B __ Created BETWEEN Feb.12,2017 & Feb.11,2018 (2017) Prices HERE.
C __ Created BETWEEN Feb.11,2018 & Oct.16,2018. (Early 2018) Price list HERE.
D __ Created BETWEEN Oct.16,2018 and March 12, 2019. (Late 2018) (Price list unavailable.)
E __ Created ON and AFTER March 12,2019. (2019) Price list
HERE.
_________________________________________
A __________
Find the full Disney price for the kind of ticket that you HAVE on the 2016 Price list HERE.
Subtract that 2016 price from the current 2018 full Disney price for the kind of ticket that you WANT. 2018 Price list HERE.
Pay that difference price for the upgrade.
B __________
Find the full Disney price for the kind of ticket that you HAVE on the 2017 Price list HERE.
Subtract that 2017 price from the current 2018 full Disney price for the kind of ticket that you WANT. 2018 Price list HERE.
Pay that difference price for the upgrade.
C __________
Find the full Disney price for the kind of ticket that you HAVE on the early 2018 Price list HERE.
Subtract that early 2018 price from the current full Disney price for the kind of ticket that you WANT on the current late 2018 Price list HERE.
Pay that difference price for the upgrade.
E __________
Find the full Disney current 2019 price for the kind of ticket that you HAVE on the 2019 Price list
HERE.
Subtract that 2019 price from the current full Disney price for the kind of ticket that you WANT on the (same) 2019 Price list.
Pay that difference price for the upgrade.