You can play on both systems.

Quote Originally Posted by Eztam View Post
removing the PS3 cd key and turning it into a PS4 cd key
true but instead of a CD-key, its a PSN code that redeems like a PSN purchase of the game and you can't buy the same thing twice off of PSN

But SE said to play on both you need, to not upgrade and buy a PS4 copy.

That could be because
1. the PS4 version costs more money and they are just trying to get you to pay more.
2. Because of the way PSN works and how they are doing the upgrade, buying and redeeming a 2nd PSN copy won't be possible.