Last month we mentioned that Sony themselves will allow their customers to upgrade certain PS3 titles to their PS4 cousins, but we didn't know how. Well, now we do. And as hoped, not only is the upgrade fee nice and cheap, you also get to hold onto the original discs.

The entire process is detailed on the PlayStation Blog. But basically, there's two upgrade paths, depending on which kind of game you nab first: physical or digital.

Those who nab boxed copies of "Assassin's Creed 4", "Call of Duty: Ghosts", "Watch Dogs", and "Battlefield 4" for the PS3 will find a code packed each copy. When the PS4 version becomes available, simply input the code and put down ten bucks; when you insert that same PS3 game into your PS4, the latter version will boot up.

It's kind of weird, the idea of using your PS3 copy to play a PS4 game, but it's also kid of neat. Especially how, if you decide to hold onto your PS3 system, you essentially have two copies of the same game. Now, if you get the digital version of said PS3 games, it's all much easier; when their cousins for the PS4 arrive, you'll simply be presented with a $9.99 price point.

Though the offer is only a limited time: codes for "Assassin's Creed 4" and "Watch Dogs" must be redeemed before January 31, 20014, March 28 for "Battlefield 4", and March 31 for "Call of Duty: Ghosts". But you can redeem those codes without having to own a PS4, so feel free to make arrangements in advance.