I suspect most writers would be fine if the changes you make
are "good" and most would be upset if the changes you make
In the pro world it is very, very rare that the script the writer
wrote and gets credit for is word for word the same as what is
on the finished product. I'll go a step farther - it never happens.
Scripts are ALWAYS changed by someone.
"Big" changes can be upsetting to a writer.
If this writer gave you the script thinking that you would shoot
it exactly as written, the writer will not be happy with your changes.
If the understanding is you can change what you want but will
not take a writing credit then the writer may not like your changes
but will be, at least, prepared.
The newer the writer, the more they are hoping their script will
not be changed at all. I always expect my writing to be changed.
What did you tell the writer?
From the tens of dozens of DVD feature film director/producer/actor/writer commentaries I've closely paid attention to - "significant" changes regarding plot lines and character introduction/removal involve the writer as per union guidelines (and the director et al is probably standing right over their shoulder, metaphorically speaking, telling them A, B, C, D changes they want); but "minor" changes regarding locations, lines, and delivery are changed at the discretion of the director/actor/editor.
On a non-union short... depends upon your relationship with the writer is my guess.
Personal politics and producer's skills.
GL. I'm sure you'll figure out something satisfactory, fingerprints and all.
(I watched "J. J. Abrams'" SUPER 8, produced by Steven Spielberg, the other day.
It had Spielberg's fingers and more all up that girl's... all over that production.
Legal precedent? I dunno. Just sayin').
If not, just take Nick out back with a tire iron and show him what "american filmmaking" is all about ;-)
For no particular rational reason I just got a disturbing image for a film scene: Twinkie freshmeat in prison is getting broken in on his first man-date and the bull is making him scream "WILHELM! WILHELM! WILHELM!" as the other subordinates in line waiting for sloppy seconds and thirds and fourths laugh.