i've done this as well, now I score the plastic with a register mark and keep a manual log of my head ranks when I replace my heads... just in case.
un-retrievable once you've changed the head rank # on your printer, but the existing head rank that's programmed in should work just fine.
otherwise, I was once able to retrieve the code by using a loop to see the "ghost" of the head rank on the printed label. Even if you wiped it clean, the original copy might still be visible to a keen eye under magnification.