I was think of an Implant Memory spell, actually.
Alter Memory already allows you to do that. You may add, alter, or erase one memory. Net hits determine how complex and detailed the memory is. The victim may roll Willpower every (Force) months to try and reduce the net hits. When net hits equal zero, the memory is back to normal.
That last part is the tricky part of the spell. 'May' implies that they have to actually choose to roll Willpower, implying that if they don't have reason to believe that they've been Jedi mind tricked, then they don't get to roll. What this means in the long term is that if you structure the memory in such a way that the change seems natural, it may last indefinitely unless they are presented with evidence of something else happening.
