As Mud said, you cannot rebind buttons in combat anymore. Blizzard made this change to prevent bots from being able to automate combat. Notably, in order to cast a spell you need to use one of the secure templates, and these secure templates only allow modification of the attributes that control what they do when you're not in combat. So you cannot have one button change spells mid-combat. Similarly, they also prevent you from modifying attributes like their position or visibility, so you cannot move buttons under the mouse either.
The best you can do is display a visual indicator of what spell should be cast, but rely on the user to actually press the correct button.