well the 10 karat means that of all of the metals used to make the band, 10 karats of them are gold. In order to truly change the metallic composition of the band, you would have to melt it down, skim out some other metals, add more gold, and then re-cast it into its original band. I don't if there's a company out there that will do that for you, BUT it may be possible that you can dip the ring in gold so that the outside just has a very thin layer. That way, all your skin would come in contact with is the gold, so that you don't have an allergic reaction, but the actual ring would still only be 10 karat.
If that's not possible, I suggest wearing like a piece of thin rubber in between your skin and the ring. You can probably get that from cutting up a condom :)
hope this helps!