This can easily be done by using game states. I recommend you read about the state pattern in programming http://en.wikipedia.org/wiki/State_pattern
In construct 2 you can do it by creating a variable named "state", set it to (for example) "playing" on start, and to "shapeshifting" when the player is in the contextual menu.
Use this variable to change the behavior of keystrokes, for example, if "state=playing" and left key is down, do this, and when "state = shapeshifting" and left key is down, do that.
I use game states a lot and I think it is very easy once you break it down to simple states. Good luck.