What exactly makes the Anakin conversion kit not work with the 07 ANH Luke?

I've recently picked up an MR 07 ANH Luke and have been looking around for conversion instructions. Of all the places I've looked, they say that I would have to reuse the original blade holder since the ROTS Anakin kit won't fit. Is it possible to take the Anakin kit, grab a bunch of tools, and resize it to fit into the ANH Luke, or am I just being a dumb? My college campus has facilities for a metalsmithing course, so if I take the class there might be a chance I could do this, or at least make my own hilt.

The inner diameter of the hilts is different, so the bladeholders for each are also different diameters. Not only that, but the cutout locations for the glass-eye, thumb-screw, and pins are slightly different on each, so they don't align properly. A couple of ambitious folks have taken the Anakin ROTS bladeholder and modified it to fit the Luke ANH hilt, but it definitely takes some heavy modification of the bladeholder if you want everything to look just right.

Supposedly Tim is working on making new bladeholders for all the Black Series hilts, which includes a Luke ANH hilt (should fit the older Luke ANH ones too).

Well, that sounds like something I should not attempt. At least with the release of the BS FFX line we should be getting new parts in the store, and I'm 64% sure they're the same molds, only different material.

You're correct about the molds. Hasbro basically reissued the same sabers as Master Replicas, only the Black Series lines have a few less metal parts, but the soundboards are a thousand times better! Only problem is, the soundboards are soldered to the LED blade-strips, so it's a bit harder to completely disassemble, and you have to be really careful when taking everything apart. That being said, the existing TCSS Vader and Yoda kits should still work (after relocating the soundboard), and Tim is making a kit for the Kylo Ren saber. He mentioned the Luke ANH kit as a possibility in an old post, but I haven't seen any recent updates.