I think the transformations is a super interesting direction to go with for Metroidvania upgrades. Giving each form their own move style makes them feel super unique and valuable by itself, and when you put them together (besides the bird being better than the rabbit and bug) you get this beautiful whole.
The only real criticism I would give is that a lot of rooms have not a lot of things (I personally prefer the other way around, when there are a lot of things in not a lot of rooms, but the design style is up to you). This is especially apparent in the fish sewer area (not the general underwater area, that one was also a little empty but kind of cool too), which contains two straight lines of rooms with literally nothing in them (except maybe a puzzle piece). Although at the same time I think the emptiness does lead to an emphasis on atmosphere and maybe that journey theme? So it's not all a bad thing.
Still, I think this is a good entry, with great atmosphere, visual colors, and movement. My personal favorite area was the rainy town. I'm excited for what you will make next!