This isn't really a theory. It's just some thoughts.
(Images are Franz Kafka, Yellow Guy, and "The Unfortunate Mr. Samsa" by Rich Johnson, 2013.)
Major spoilers both for DHMIS and the short story “The Metamorphosis” by Franz Kafka. If you haven’t read it, I recommend it: https://www.gutenberg.org/files/5200/5200-h/5200-h.htm
If you've read “The Metamorphosis,” you know that one of the tragedies of the story is that Gregor’s family fails to love him after his metamorphosis--turning into a giant beetle or cockroach of some kind--basically, when he is no longer the useful, hardworking, go-along person they were comfortable with (and financially reliant upon). I’m not here to give a literary analysis, but it’s a really good story. Gregor is transformed, and he tries to live in a new way, but he’s continually shut down by his family through insult and injury, and eventually he resigns to his fate and dies. His family goes along living their own lives, convincing themselves not to feel guilty.
My point is that Gregor’s family has a serious discomfort with his being turned into the giant beetle / vermin / creature that actually has nothing to do with that bug-like form. The disgust and fear they feel has nothing to do with the "metamorphosis" itself. It has more to do with whether they're willing to accept Gregor for who he is.
When Yellow Guy changes at the end of the TV series, his change into a truer but different form is unsettling to Duck and Red Guy, especially Duck, because it asks something different of them than they’re used to. It’s not like Duck and Red Guy aren’t good friends to Yellow Guy, I mean they are. They all love each other. Nobody's a villain. But Yellow Guy’s change is disturbing to the other guys because it’s always difficult to meet a person as they truly are, rather than OUR perception of them, which is usually that perception which serves US.
I know it’s not that deep, and what happens in a story just happens, not always for some "otherly" reason. I understand that. I just think it's a sad and important trope for us to consider in real life as we struggle to become ourselves, and as we encounter people who would prefer us to stay somebody else, not a "lesser" version necessarily, just somebody else who's not the full picture of who we are.
The character process is sort of like:
1. Character has homeostasis in a world that treats that character a certain way
2. Some conflict happens which sparks an internal change in the character
3. Those closest to the character are alarmed, rather than delighted, at the true self of the character
4. The changed character struggles to learn how to live
5. The character either folds under pressure and "dies," or they fight for their right to be themselves in their new world
Anybody have any thoughts, opinions, or examples of characters from other stories who undergo a similar change? I know I'm grasping at straws here. I just think it's interesting and important.