Piece of cake. If you stay with the Std Mag .535" bolt face, it's all you have to do. For instance, after the mag bulkhead is removed and the bolt stop slot is lengthened, a .300 Wby becomes a simple re-chamber job. A .340 Wby or any of the other long case (.535" head) calibers is a re-barrel job. The action will also take any of the Rem Ultra Mags, but you will need a single stack mag box and follower, (available from Wby). If you go to a .378-based cartridge, you will have to open the bolt face and change the extractor, in addition to using the single stack mag box and follower. The floor metal will work as is.
These mods are easy to do on a MK V. If you re-barrel, depending on the new barrel contour, you can use the same stock too.
Oh yeah, if you want to go to a .257 Wby, .270 Wby, 7mm Wby Mag, or any other std length Magnum, all you have to do is just rebarrel. No mag, bolt stop slot, or bolt face mods needed.
Just my opinion, but the MK V is too good an action to waste on any of the short magnums. It's long and strong. Use it for that.
Have fun, Tom