A Mary-Sue was a process that an individual elected to have done, it was the process of wiping the individual's mind and then rebuilding a wholly new individual onto the individual's mind. A Mary-Sue was also the process of placing an a new personality profile over the top of an existing individual's personality, making the process reversible.
Bernice Summerfield underwent such a process to protect her mind from the All-High Gods on Dellah when she returned to retrieve her cat Wolsey. The process that Bernice underwent was reversible. (NA: The Mary-Sue Extrusion)