Variable definitions form a hidden “PreCreate” event - even though it exists in some code-like form at some point during compilation, GameMaker doesn’t really let you anywhere near it, less so break out of it (which you kind of have to with if live_call() return live_result).
For purposes of playing around with variables, the common way is to have a live-coded Step event where you change those and move the values to their original spots after you’re done.