It's most likely doable, but probably cost prohibative. Depends on which brand of rifle, barrel contour, do you want a magazine or is single shot ok.
First you'll need to change the bolt face diameter, on a Savage it's a relatively simple bolt head change on the others you need to machine open the boltface.
Although the wssm case is large in diameter it is considerably shorter than the 243 so you'll need to cut some of the chamber end of the barrel off then rechamber and rethread and in order to do this you have to have enough straight area before the contour of the barrel changes to accomodate the new threads. This of coarse will change how the barrel fits in the stock, probably won't be as aesthetically pleaseing to the eye.
If you have to have a repeater you need to swith out the mag box and follower and may or may not need to have some feed rail work done.
Like most things in life it's just never as easy as it should be.