That's a dumb way to do things for a fighting game. What you should be doing is spawning hitboxes as a different object which has frames/animations to choose a shape. This way you can make them move independently of the character, have them shoot out, etc. Not to mention it's a thousand times easier to tweak (which you will be doing nonstop making a fighting game) a position/offset in code, than to open up the image editor and drag points around.
I can't think of a use case for your original idea either. Why would you want to change the collision mask of an object without changing it's image? It's just confusing to the player.