As a comment to the original topic, a SE rep answered this question on Reddit

Q: If I have both PS3 and PS4 licenses in the same account, but I only have Heavensward for PS3. Can I still upgrade the license so I get Heavensward for PS4?
SE_Kahuna: Just had a quick chat with a colleague about this - you should be able to upgrade without any trouble and then get a code for the Heavensward expansion to use on the PlayStation Store. So yes, possible! But please do remember that you will immediately lose access on your PS3 for the entire game.
https://www.reddit.com/r/ffxiv/comme...rade_campaign/