The moment you want to make it so that what _I_ wrote is something else,
That's not only legally not allowed, that's a plain impossibility.
It is perfectly legal to modify what one has written, this is what the GPL does, it grants such a right.
Nobody can "make it so that what [you] wrote is something else", [...]
Sure they can. I write a program looks like this:
(if (give 'ams 'cookies) (setq ams-happy t) (setq ams-happy nil))
Lets translate it into English: If ams gets a cookie then ams will be happy, otherwise he won't.
So now you modify this with s/cookies/death/, and viola. What I wrote was modified into something that is simply not true, yet still allowed if such permission was given.