I have quite a lot of respect for Jim Williams but must sadly confirm from experience what has been pointed out above: Modified microphones can be quite difficult to resell at all, let alone at a price that lets you recover what you paid for the mods. The number of buyers interested in such items is far less than those seeking the stock item.
Furthermore, the people who want modded microphones tend to be quite choosy. If you had your mikes modded in 1996 they'll insist on the version that became available in 1997. If you paid $400 extra for option X, they'll bid less even though option X can be switched off entirely, because they don't like that extra switch on the circuit board--and so on, and so on.
At some point I should really tell my whole story of sending three kinds of microphones to three different microphone "modifiers" including Klaus Heyne and the late Stephen Paul. My experience with Jim Williams was quite good as far as I was concerned, but I decided to sell the microphones (a pair of AKG C 414-B ULS) on eBay after a while, had to place the ad more than once before I got a single bid, and took a loss of several hundred dollars on the deal. On the other hand, I completely suck at selling things so YMMV.
--best regards