Hi,
Why is the duration sun? I would have expected a momentary duration would change memory permanently.
Best
Jan
It's a Muto spell.
Cheers
Mmh, ok... So changing memories permanently is impossible? A pity... Anyone knows a Bonisagus who is working on that particular limit of magic... 
On the other hand...
Not every change is a radical one and with care things can be made quite plausible...
No, not impossible unless you want to be technical and restrict it to Hermetic magic.
With Hermetic magic you can do the following: PeMe to erase a memory, and that is permanent. The CrMe ritual to create a permanent memory in its place. Now you have a permanently changed memory without actually having changed it. Sure, it's like you asking for someone to repair a sword and they swap it out for a different, identical one in good shape, so did they really repair it? But on the receiving end, could you really tell if it was repaired or swapped out?
Now, if you're willing to avoid Hermetic magic, you could use Solomonic Storytelling.
I believe there's a discussion in HOH:S Jerbiton section discussing how to create longer lasting effects with fleeting memories. For example, giving somene a Diameter duration memory then talking to them about their memory for a few minutes, then walking away. The created memory will fade, but they totally remember they promised to deliver that sack of grain you purchased last week.
Found it at HoH: S p. 68ff
Doesn't really talk about Muto though, only CrMe that keeps traces after duration ends.
You can probably get a similar effect with Muto - changing a faint memory of "not doing something last week during business hours" to "selling a bag of grain to a magi". Then have the same conversation. When the muto effect wears off, they don't specifically recall the sale, but they still (as with the Creo effect) recall having the converstaion recently.