It's not illegal to possess large capacity magazines, no matter how they were acquired. It's not illegal to buy them, but it is illegal to import them, give them away, or sell them. It's not illegal to keep them if you somehow find them. If you have a large capacity magazine for a gun model that was produced only after Y2000 you may have some 'splainin to do.
If you owned them before Y2000, it's legal to repair them, including replacing each and every part with a brand new part. There's no requirement to retain the old pieces to prove you did this, and you don't have to match brands. For instance that rusted out Vietnam-era M16 mag you've had since the 60's can be repaired using Magpul PMAG parts until all you have left is a brand new Magpul PMAG. There's no date stamping or LE Only stamping requirement.
You can buy "repair kits" which are just completely disassembled magazines. This is legal. You will find these offered on calguns.net from several vendors.
If you are the proprietor of an armored car company, you may buy and sell large capacity magazines at will. Makes you wonder why there aren't more Armored Car companies springing up in the Golden State.
Finally there is a 3-year statute of limitations on manufacture and import.
This is just a summary. Read this for more:
http://www.calguns.net/calgunforum/showthread.php?t=124709