I will put forth my own version of the argument that "whatever is changing is being changed by something already actual (real, existent)." The original version from the Summa is here:
"...it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another."
1. A potential does not exist (yet)
2. Something non-existent cannot cause an effect, such as making itself actual
3. Therefore, a potential cannot cause an effect, such as making itself actual