Well the .243 case is pretty close to the .308 in those areas that are critical as far as the feed lips are concerned, it should be no problem rebarreling it, but money wise you'd probably come out better by trading it in on a .308 of similar vintage and book value.
There are probably more .308 owners that would like to trade for a .243 than vice versa. Though with the tendency towards folks looking towards military calibers these days I could be wrong. It depends mostly on the type of big game in your region.
I've only owned one .243 , a Remington 788, and kick myself (pause to kick own kester) whenever I think of how I sold it expecting to find another in .308 as easily.
I'd been intending to take advantage of military surplus 7.62 NATO then I'd read up on warnings that many commercial .308 chambers and throats optimized for sporting rounds didn't work out that well using the military spec cartridges.
Apparently wartime pressure on manufacturers has driven up the price of good 7.62 ammo, and theres a boatload of truly crappy 7.62 NATO ammo from foreign manufacturers out there as well.
I broke down a sackful of cruddy looking South American 7.62 ball some years back and the square flake powder somehow broke down into an extremely corrosive gas that more or less dissolved the steel powder canister I stored it in. Even the gilded steel core bullets began to peel. Though I got those rounds for free they were of no use at all, and I'm lucky the powder did no more damage than to cause every teel item stored nearby to rust like it came from a shipwreck. Could have been much worse.
If I have any of those cases left laying around I'll post the headstamps so the folks here can spot this dangerous ammo.
Also some Spanish(?) made Santa Barbra ammo was found to generate proof load type pressures and wrecked a few of the Spanish 93 and 98 Mauser bolt actions that were rebarreled for the NATO round.
Basically all this amounts to is that surplus ammo is no longer a factor in chosing the .308.
Be nice to run across one of those Remington 600 Carbines in .308, almost got one of those when they first came out, but I was dumb enough to pass on it when the gunshop owner told me that due to ammo shortages in Nam at the time no surplus 7.62 was available locally.
Plenty of great rifles in .308 available these days. I'd suggest that you look for one whose action is best suited to the OAL of the .308. Its length is its only really notable advantage when compared to the .30/06. Short throw Bolt actions, and the less common leveractions, suited to the round can be handy and compact.
