Asking this just so I have the most relevant directions on this

I have been playing a steam purchased copy of FFXIV for a while now, and plan on rebuying the game on PS5 to be able to play it on there. I plan on getting the collector’s edition on PS5, so I can have all of the mounts from the previous collectors editions.

I already have a reoccurring paid sub, however I am confused as to the whole service accounts mess. I was told to be cautious while registering the PS5 version in order to avoid accidentally creating a new service account, and was wondering as to the process of logging in on the PS5 and registering the game to my current account once I buy it.