Swap the two pins on the harness as necessary. I had to do this same thing, except with my headlights (which use the same plug actually). It's fairly simple to do though with the existing plug -- unplug the relay, get a small flat-head screwdriver (the size used for eyeglasses, small electronics, etc.), and press it in from the rear of the harness, the side the wires are on. Then pull the wires out. Do NOT use force, it should come out easily.
Basically there's a small "barb" on each metal spade that holds it in place. Pushing this barb in so it's flat, the pin slides out freely. Then gently push the barb back out with a screwdriver and re-insert in the new location. You'll have to do the same thing to your headlights if you ever upgrade from sealed beam to H4, for the same exact reason (pinout is different). As long as the plug is intact and the relay is the right size, you should be able to swap wires using that method.