Now you might say "you have a lathe, dummy! Just turn the shank down! Piece of cake!" Alas, the end mill is carbide. If I tried to cut it with even a carbide tool, I would ruin the shank, certainly the tool, and maybe even the lathe. Not an option!
The only way to do this with my equipment is with a toolpost grinder. But nobody makes one for the Taig. So I improvised one. I started with some 1/4" - 6.3 mm steel square bar from Lowe's. Nice nickel finish on it, too. The Taig toolpost takes this size. Ten second's worth of time with Handy Bandy cut it off. Then I milled the ends square.
But it worked. The new shaft fits, just, the 3mm collet, so I have a 4mm endmill at my disposal on Cecil B. de Mille.