Bullpup,
There are many things to be considered when attempting to have a rifle rebored.
1st: If one is going to a chambering that is already available, then it is cheaper to sell your rifle and buy another.
2nd: If one is going to bore then the groove diameter (max depth of the groove...) of the intended must be greater than the bore diameter (Max height of the lands...) of the donor barrel. Thus, I've never heard of a custom bbl maker that would bore a 6mm to .25 cal. I believe that two calibers is considered minimum.
3rd: The intended chambering must be equal or greater than the case of the donor. In the case of the .25-06, the '06 case has greater taper than the donor (.243 Win.) Therefore, your options are limited...
A .243 can be rebored to .260 Rem., 7mm-08, .308 Win, .338 Fed, or .358 Win. (Case and headspace are the same, only difference is bore diameter...
Also, One could rebore it to any magnum case larger than .25...
i.e: .264 Win, .270 Wby, 7mm, .300, etc... up to about .35 cal.
Or any of the "Short Mags"
In the case of the .25-06, .308 Win, or 7mm Mag, one can buy the same rifle so chambered, so it would be cheaper to sell yours and buy another...
Generally the cost of reboring is around $225-$350 depeinding on who does it. And at least a 1-2 yr wait. Due to the set-up time, most reborers like to do them in blocks. IOW, they don't like to do them one-at-a-time. They like to have several (Of the same intended bore/groove dimensions) to do at a time.
I have a #1 that was a .22-250 that I rebored to 6.5/'06 (Actually, 6.5/.270... same/same except it has a .050 longer neck length.) and I have another that was a .270 that I had bored to 9.3x62. Really the only reason to have one bored is so that one can have something that is otherwise unavailable.
The other advantage, is that you don't have to mess with the inletting on the stock.
Hope this helps.... Grasshopper