You mean change animation not behavior.
Make a global variable, name it any thing, for example "act"
Make event like this:
System - every tic
(Sub event) Act = 1 ------------- set player animation to monkey
set jump strength to xxx
etc........
(Sub event) Act = 2 ------------- set player animation to cat
set jump strength to xxx
etc........
------------------
And make this event
When space key pressed set global act to 1
When x key pressed set global act to 2
Etc............