very easy! all you need is a different magazine box and barrel and you are in business!! i have set up one of my 700's as a switch barrel rig and one of the barrels is a 338 rum... i do like it but i haven't shot it a lot as the weather and my schedule have been conspiring against me for the last couple of months.
brownells carries e.r. shaw barrels in the 338 rum that are prethreaded and shortchambered at a very good price. you should be able to get the magazine box and barrel for under $200 and another $100 to install, headspace it and blue it.