Also from a really pedantic argument we got into on my cube discord, it's possible the rules actually only allow for transform cards if at least one half of the card is a permanent. No judge has weighed in yet, and as far as I can find it's not forbidden, just not mentioned.
Though that is me interpreting the following:
Code:
712.5. Only permanents represented by transforming double-faced cards can transform. (See rule 701.28, “Transform.”)
As "permanents that aren't DFCs can't transform" not "only DFCs that are permanents can transform"
Also, while the rules explicitly and exclusively talk about "transforming a permanent", there is actually already an exception;
Startled Awake is a non-permanent card that transforms (into a permanent). So, I believe that the rules don't mention non-permanents transforming because the mechanic usually only works on permanents, since an instant or sorcery doesn't "live" on the battlefield. There is precedent for the mechanic working on a non-permanent spell, even though the transformed side
is a permanent, that doesn't detract from the point that the front side of
Startled Awake being a sorcery does not match the actual rules. Additionally, the disturb mechanic definitely changes what's possible, as this marks the first time you can actually
cast the transformed side of a card, and that definitely
can work for instants and sorceries. They just haven't tapped into that because it a) doesn't work with the intended flavor (creatures that come back as haunting spirits), b) breaks the precedent set by earlier iterations of the transform mechanic, and c) is pretty annoying to play with because you have to desleeve and resleeve the card in question within a matter of seconds just to show what the backside does, but then the instant/sorcery has to go to exile (in this case) with the front side showing again.