Sorry, x --> y is for change x to y. With robot they aren't all the same, some are mecha, some are other things.

I know strike witches aren't mecha_musume or cyborg, but I dont know any other suitable one to use for the machine being separate to the girl.
The reassign was based on content, mechagirl is full of images where the girls are separate to the machine, some even look like just armour. But if you made the tag that was for girls that are fused to machine into mechagirl then a lot are going to need to need retagged.