I really hope no one has beat me to this one but if they havent well here it is enjoy and hope you save money.
Ok to all that might not know and want to jump cross platform. If you already have a copy of either Rise of Zilart and CoP and wanna go to PS2 for whatever reason is more expensive than to go from PS2 to PC but you still save $30 dollars.
Ok the way to do it is this get a copy of the game for PC from a friend and install it. When you are pompted to register input the codes of the version you own i.e. PS2 to PC. Unfortunately you need to have the hard drive to jump from PC to PS2 so that will cost you $100 dollars (maybe less I bought mine at launch)input your PC codes. Get a copy of the PS2 version of CoP and input your PC codes for that version. Presto free cross platforming.
Remember to always have a code for each version, meaning you have already bought the game for either platform.