Basically by upgrading to the ps4 version, your ps3 version becomes unusable. The cd key registered to that CD was binded to your account, by transfering to PS4, it is basically removing the PS3 cd key and turning it into a PS4 cd key. You can play the same account on both systems from what I understand, but you would need to purchase another copy. Same way as those of us that play on PC, can still play our characters on PS3 if we purchase a copy for it and register that key to our account.