It depends on threads you are changing to and from. I have retapped 5/16-14 threads into a Predator shaft with 5/16-18 insert. The guy was wanting to use it on one of my butts without having to get a new Predator shaft and after about a year he is still going strong with it. No Problems. Now to change it to a 3/8-10 thread from 5/16 would be harder, but not impossible. What thread do you have and what do you want to change it to?
Chris www.internationalcuemakers.com
This question is actually very popular today with the new Z shaft only being offered in uni loc joints. Just in case that's what your referring to.
Remove insert, plug, drill, bore, thread, and fit. Not to bad if the shaft is straight.