So I’ve been debating getting the blizzcon collection this year, but one of the features I already have (the OW2 S7 BP). I opened a ticket asking about it and, as of writing this, you do not get a discount or reimbursement in cash, BNet Balance, or even gold OW coins if you buy it after owning the Battlepass.
The quote from the GM:
Greetings,
Currently, there is no discount for already having the Season 7 Battle Pass when upgrading to the BlizzCon Collection. For any feedback about this, please let our developers know via the forums. No personalized response is guaranteed just as a heads up.
Sincerely,
Specialist Game Master Ritorack
Customer Services
Blizzard Entertainment