I have a Bowie knive which is just too big and unwieldy at 15 inches , but i dont want to go through the hassle of selling it and buying something smaller [and losing money on it]i would like to shorten to 12 and a half inches .What is the best way to do it without ruining the temper etc ? thanks

Use an abrasice cut-off wheel the grind to shape .The important thing is to not get it hot .Grind slowly and dip in water frequently so as not to develop colors on the steel.

I suppose that may work if the blade was completely heat treated, you will change the sprofile of the point at the very least and if it was differentially heat treated the point will be soft.

Keep what you have and buy what you need. You'd "lose money" if you shorten the blade because you'll end up destroying the knife.