The switching in this feels like a cool theme to use for a level or group of levels in a platformer instead of an entire game based around it. I could see this idea being used for a Mega Man level theme, with a boss that also switches in/out somehow.
I'd like to see the player walking and jumping accelerate faster. Right now, it doesn't quite feel like moving underwater, but it's a little closer to that than it should be.
Your first room confused me for a bit. I suggest altering the left side of the room so the exit is narrower and more visibly hallway shaped when viewed from the starting room. I didn't realize there was a passage there at first, assuming it was just another wall.