You would be best off asking this in the cue makers section of the forums but for now...
1) Yes in most cases (if not every case) the pitch can be "changed" (that being the threaded insert can be replace, or the wood can be plugged and rethreaded). Be prepared for the change to possibly alter how the shaft would feel; though maybe not enough for you to notice or care.
2) I would expect cost to vary based on who does the work, the thread-type you want to change to, and if it's a metal insert or wood. A few years ago I saw a few quotes for around $60... But those are old quotes, I don't know if they were reasonable quotes. And without any info on what your requirements are, you can't assume somebody else's quote will apply to you.
Radial to a 3/8x10 swap for a wood joint should not be much at all, I'd guess $40 for just a simple fix and maybe $60 if you are going from a wood to a metal insert.
Don't forget that when changing the threads in a shaft, you will also have to remove the pin in the butt. and essentially do the same steps on the butt as the shaft.
Add the price of an insert and pin to that. Radial pins are a bit more costly than others.
Just to give a little clarity to the situation:
I came across a shaft that is to my liking: short ferrule, conical taper, 12.5 mm.
When I got it, it was told to me it was 5/16x14 thread.
It is not.
I have a butt with a 5/16x14 pin.
I wish to have the shaft thread 'converted' to fit this particular butt.
No conversion on the butt is necessary.
By the way, thanks to all responses.
Keep 'em coming. :thumbup:
Wutang