The Square Enix account will become locked to the PSN account associated with it. So if my PSN account name is (ILoveSony) and it is tied to my SE account (Realm Reborn), then I must be logged into ILoveSony to play using account Realm Reborn. You can use any PS3 but it must be active with your Sony login. I have done this with a friends PS3 so I do know it works.

In answer to your question, yes you must buy a new copy for the platform you wish to play (and yes correct region is important). After you add it to your account you will see it in the Mog Station. Here is mine.



As you can see I have added both PS3 and PS4 to my WIN account. If you don't care about using a PS3 after you get your PS4, you can upgrade for free but you will lost access to the PS3 version.