Find Floor, how it works, how can I override it?

Hello guys, Im working on a 2D game, and Im trying to implement a gravity change, until now the only thing is working about this is changing the gravity scale of my character movement, but even though the gravity is oposite, he still thinks the floor is downwards …is there any way to change the “Floor” dynamicly?

Does the “Find floor” runs once at the start of the level? …can I call it back in the middle of the game?