You can always revert to reforming .375H&H if brass becomes unavailable for the .300H&H, which I doubt in the next 5 yearsor so.
: For re-forming, you may want an intermediate die, like a .358 Normal FL or seater die, then try a .300 H&H seater diek then the FL die. You will have to lube the cases, but barely. Too much lube is usually the problem.
: For now, search around and buy up what you can. I wouldn't be concerned.