I don't know about the conversion kit, but I did buy the bellows stage from a newer Beseler 45MXT on eBay and installed it on an ancient Beseler 45M. It simply replaced the aging tapered bellows that were there, no modifications needed at all, just disassemble/reassemble with the new parts.
FWIW, though, I prefer the tapered bellows stage on my older blue Beseler 45MCRX because the lens stage can be tilted to do a bit of perspective correction when needed. The larger bellows on the MXT can't. Plus, it takes about five seconds to change lenses with the spring-mounted lensboard. The turret would really only be handy if one were mass-printing lots of prints from a different size negative each time. I don't use my turret at all; it lives in the cupboard.
Best,
Doremus