Why ????
In my "neck of the woods", you can get a good used 760/7600 in .308wcf for less than the cost of replacing a factory barrel on your .243.
By the time you factor in the cost of the barrel, even if you only pay $50.00 for it, then take or ship it to a gunsmith, pay labor at $50-$75hr, return shipping or gas at ~$4gal to pick it up, ect. You'll have more in the rifle than "another" one would cost you.
And, you'll have an extra magazine to boot. Also, you can put a higher power scope on the .243 for "longish ranges", and a lower power scope on the .308wcf for "thicker" areas with shorter ranges...
Just my opionion, as I "been there, done that" with other rifles/calibers.........
Besides, the .243 actually works better at longer ranges. I've had more close range bullet failures with the .243 than anyother cartridge. Due over expansion, and under penetration.
Actually, unless going after elk, or simular sized or larger game, I'd just stick with the .243......... Use enough bullet, however!