This is exactly why all those masters should have been based on a "master" master. If they have, well the answer's rather obvious, if they haven't no you can't but it might be best to rejig it so they are now.
I see what you mean. Unfortunately I'm not the one who initially created the document...but I think I found a work-around
What I did was create another master, setup with my desired guides and margins. I then dragged my new master to the existing masters and they all updated with my guides and margins.
Thank you for your response!