Just about any repairman and definitely, any cue maker wil reduce the size of the shaft for you.
You can have it done all at once if you are in a hurry but no one will give you warranty on the work if you do it that way.
Its just too iffy and they might warp. I had 2 shafts done in Vegas, from 13mm to 12.5 and 12mm.
I was lucky as neither one moved to this date. You may not be so lucky.
To do it properly, you will have to leave the shaft with the cue maker for
how ever long he wants it to do the job. He will take it down a bit and let it rest for a month before taking any more off.
If you happen to buy a cue with 2 shafts, play with one and let the cue maker have one. Or, buy a used Predator shaft and use that until your shafts are done.
If you look at some of the cue makers that have web sites, they usually include the charge per MM for taking a shaft down.
If you ask this question in the Ask the Cue Maker forum, several people will probably send you a PM with a quote.
It depends where you live but your best bet is to send it to someone fairly close to you.