Re: 300 Win to 300 UM?
It is an easy job. However, if your going to spend the money to get it re-chambered and send it off to a gunsmith or take it to one you might as well spend the extra cash and get a quality barrel in a twist that matches the weight bullets you intend to use.
My suggestion for a particular barrel maker are Shilen, Hart, Douglas, and Lilja, all are high quality and are in no particular order any of them will perform exteremly well.
Since this is the Longrange board, why not take the leap and get yourself a long barrel and send it off to have the 300 Tomahawk chambered. Youll be right where you need to be when working with it using information gathered on here.
Good luck and happy shooting.