I own both of these guns, flanged in a double rifle and bolt gun for the H&H version. I have hunted both in Africa and elsewhere, I also reload both of these rounds. Along the way I have learned to never tell others what to do with their money, guns or wives. So do as you see fit with your guns.
Please note however that these are in fact two different cartridges released by Holland & Holland in 1912. The shoulders are different, 12 degree and 15 degree Flanged to Belted respectively, the shoulders are located in different positions on each case, the OAL is different, and the belted is generally a little more potent, 100 fps, over the flanged. I personnally would never try to load either case with anything other than the exact reloading equipment for that individual case. Notwithstanding the obvious concern I would have for safety, the possibility of a fouled cartridge in the rifle is simply to high. Additionally, double rifles are regulated to shoot a specific load, if you alter the load you will most likely change your impact point on your target, which while a lot of fun to shoot through, it can be a bit tricky to work up a handload that regulates well in your rifle.
Reloading dies are readily obtainable for either of these cartridges from RCBS amoung others. Subsequently, I would not personnally risk damage or injury trying to interchange one for the other.
Good hunting...
