The Black Mage 60-70 quests also had an instance of memory transference. The Mhachi mage who first came up with Black Magic set up a spell that transferred a copy of her consciousness into one of our quest-line friends, and would occasionally take her over now and again. If I remember correctly, it started out as amnesia for the possessee, but she eventually started becoming cognizant of what was going on when she wasn't in control of her body.
It was only meant to last a short amount of time, and the transference wasn't permanent. But preserving and implanting new memories, or even entire personas, can be accomplished with a simple application of magic.