So, for example, is it possible to make the character go from walking regularly, to all of a sudden walking on walls or the ceiling? I'm not asking for any help or code, I just wanted to know if it is possible or not before trying to figure it out for myself. I don't really want to spend hours searching just to figure out its impossible to do that with Roblox Lua.
Yes, and no. You can't change the gravity like that, BUT you can script it. Instead of changing the gravity, you could use a body force to keep them up to the ceiling, and change the characters movement to allow them to not glitch off. Another solution is to just rotate the thing they are walking on. Works great singleplayer, if multiplayer, others might need to be invisible.