The answer is probably out there, and I have overlooked it.
How long does sustenance last you?
This isn't just a question of drinking Moon duration CrAq water for a week, then have two weeks of natural water before the Moon duration finishes.
It started off with a hypothetical of MuAn a horse into a fish. The fish would drown in air, then the corpse of the fish would revert to a corpse of a horse.
The somebody asked, what happens if you ate the fish? My answer, if you convert a 1000lb horse to a 10lb fish, cook it and eat it, then it reverts to a cooked and masticated horse in your stomach, and you had better hope you have a larger enough stomach.
Then they asked, what happens if the spell duration is long enough, that you have time to fully digest the fish before it reverts to a horse.
And I am not too certain of the answer. You get extra sustenance?
Then we got into questions of using a Moon duration spell to change a 0.1lb minnow into a 10lb cod, and eat it. Do you suddenly have 1/100 the Sustence you thought you had at the end of the speel duration?
The issue is fundamentally one of game balance versus narrative.
In most stories if you change the horse into a fish and eat the fish, you have eaten a fish, and get the nutrition from eating a fish.
In terms of game balance however most people will instead change a mouse into a cow then cook and eat the cow, which means getting a lot of food for free.
Food which is magically created without vis has no lasting value- you are eating nothing, but the rules for muto are far less specific. You could go with either extreme or somewhere in the middle- eating a horse turned to fish won't erupt in your stomach but will sustain you without feeling hunger as if you had eaten a horse. If you go with this plan however I would probably rule that you are 'digesting' the essential nature of the horse that is in the temporary form of a fish and thus the duration is irrelevant (unless it ends before you finish eating...)
I was considering similar. For example, what if you shrink yourself down to two inches or five centimeters tall? At that size you consume some non-magical food/drink. If you stay that size, do you only need to consume a very small amount of food, thus making your rations last longer?
And there's no winning here for 'balance'. If we rule that you need the food and water from your normal size (and species!), then shrinking someone for a month to a size where he cannot consume all the food and water he needs will provide a nice and tortured death. If we rule that you don't, then we can shrink the peasants for the winter.
I'd want to rule that you need the nutrition of your new size and form. Otherwise, a person transformed to stone for a few days will die of thirst. I think that's worse than petrified peasants who don't need to drink.
There's a variant of this in Magi of Hermes (pg 10) - Alexander of Jerbiton transforms his grogs into bats, they eat insects and then a well-nourished bat transforms back into a well-nourished human in the morning.
[Take with the usual caveats people have about Magi of Hermes.]