Record them as is onto separate tracks to your DAW, then use something like Drumagog to replace the samples.... probably a lot easier than trying to replace the onboard sounds....
Well - ok, my solution is somewhat impractical without having a DAW... OTOH, I think you'll find it quite difficult to replace the on-board sounds. Without a DAW or h/w sampler (that you could use midi to trigger alternate sounds), there's not a whole lot of other options!