A few notes about Hi-caps.
It is not legal to sell, lend, loan, borrow, make a hi-cap, import after 2001*
It is legal to buy**, own (does not matter what date or model, can even be marked LEO only), find one laying in the street, repair a hicap with hi-cap parts.
IT is legal to replace all the parts in a hi-cap mag with hi-cap parts. Does not need to be the same martial, plastic vs metal body.
You CAN increase a hi-cap to a higher cap. Example 12 rd mag to a 100 rd mag. As far as the law cares you owed one hi-cap and you still own one hi-cap.
You do not need any proof about when you bought, given, inherited your hi-cap
*not 100% sure on the start date, i just know that its in effect!
** There are two thoughts on this, the law does not say it is illegal which means its legal. But this is not something to test in court.
Calguns.net has a nice thread with a question and answer on what you are trying to do let me find the link
http://www.calguns.net/calgunforum/showthread.php?t=81127
Good luck!