When you bleach a print in rehalogenating bleach, the bleach acts more quickly on grains of a certain size, removing them from the mix, so to speak. The bleach affects the size and surface characteristics of the remaining silver grains in the print. The result with extensive bleaching is often a marked shift in image tone. MCC-110 seems to show this quite readily; I have to be really careful not to overbleach when doing localized bleaching with this paper, or the bleached area will change tone too much.
The bleached image, which is no longer visible, has been changed to silver bromide (a light-sensitive silver compound that is now fully exposed since you bleached with the lights on, but still undeveloped) and can be fixed away just like any other undeveloped silver halide compound. Refix and rewash and you'll have a permanent print.
Doremus