Lets consider i've a following page
This is safe because it means that any other code that is calling these functions will not cause errors - they just wont do anything.
Give it a go and let me know how you get on.
Edit: I have confirmed that this approach should work. Here I switch the function from Does Something, to Does Something Else and back again.
results in 3 alert boxes in this order:
Does Something Does Something Else Does Something