I'm part of a FFXIV community on PS4 and a few members have had this same issue with the only difference being they wanted a new square account linked to their current PSN. You seem to want to link an old square account to a new PSN. From what I remember of what those members said, when unlinking the accounts, Square Enix deletes the square account that is linked and attaches a new one to the psn account. So I don't think they can actually help you since you are using a new PSN. I could be remembering wrong, currently asking one of them for details. not sure if/when they'll respond though.