That doesn't work if "the highest part of the list" is above SM2:

The new creature must be an option from a spell of the same level or lower as the spell that summoned the target.

The only way to make that work would be to heighten the spell to an appropriately high slot, but then you're back to using high level slots. Without heightening, all you could do would be turning your SMX creatures into SM1 or SM2 creatures.

That being said, there is a standard abuse formular for this spell (which is why it's, to the best of my knowledge, outlawed in PFS, and you can safely assume that any GM you pull this technically legal move on will shut that shit down right quick):

You need a heightened ASM (heightened to your highest available spell level) and a long duration summoning spell (Mount or Phantom Steed for example), then you cast your Mount, change the horse into an SMX creature and end up with an hours/level summoned monster.

Again, legal but abusive, if your GM has any sense, don't even try it.