I'm not a lawyer, but yes, I do know the answer to this.
It is illegal to manufacture new high capacity (over 10 round) magazines. Period. End of sentence (or begining of a possible prison sentence if you do...) This applies to both manufacturers and individuals. The exception is mags manufactured for military or law enforcement use, or those intended for export to other countries where they are legal. Those mags are almost always marked.
It doesn't matter if you create your new high capacity mag by "converting" an existing low capacity mag through parts subsitution or by assembling all the individual components into a new mag. Any way you look it at, it's "manufacturing" and it's illegal. Even if all the individual parts existed before the ban in 1994, they had to have been assembled into a mag before that date to be legal.
It is also illegal to "create" a new hi capacity mag from a low capacity mag by substituting different ammo than the mag was designed to use. That means that it is illegal to load 12 or 13 rounds of 9mm ammo into a 10 round .40 mag. (Now if the .40 mag was preban AND originally held over 10 rounds of .40 ammo, it would be ok. With either ammo it's still a preban high capacity mag) I know some here disagree with me on this, but the only way I can "prove" this is by getting a letter addressing this specific issue from the ATF, and I'm not about to poke my hand into that hornet's nest.
The only high capacity mags that can be imported are those that were manufactured before the ban in 1994.That means that for some weapons, there are NO civilian legal high capacity mags existing. (Certain HK pistols designed in Europe come to mind because even though they were designed for high caps and are sold that way in Europe, no magazines were actually produced before 1994)
Replacement parts, like mag bodies, can be manufactured and imported. They are intended to be used only to repair damaged or otherwise unservicable pre ban high capacity magazines on a one-for-one basis. (I hadn't heard the bit about only the factory being allowed to do those repairs so I don't know if that is true or not)
Are people illegally assembly high capacity post ban mags from replacement parts? I'm sure some are. Even if some people seem to be getting away with it, it's never a good idea to break the law or instruct or aid others in breaking the law.
Trebor