I was wondering if this would happen after the same thing happened on PS4 for PS+ games that went Cross Buy afterwards and it does work the same. No more PS+ required after grabbing the PS5 upgrade. Not sure if it works for all regions, but it should for at least NA/SA.
PS5 Upgrade: https://store.playstation.com/en-us/product/UP3509-PPSA02048_00-DEADBYDAYLIGHT00
PS5 Upgrade: https://store.playstation.com/en-us/product/UP3509-PPSA02048_00-DEADBYDAYLIGHT00