Hey guys, thanks for the reassurance; as it turns out I was trying to do it in the exact same way as you described but it didn't work - because the patch I was trying to overwrite was protected!
I didn't realise the flashing 'protect' lock icon meant that it was write-protected, I figured it was just telling me I couldn't save a USB patch to the User location - silly me! I un-protected it, tried again, worked like a charm.
Thanks again!