editing php of plugin

This is actually a simple question in regards to plugins in general. I have been developing websites using WordPress for over a year and have had to edit some plugins to fit the our clients specific needs. Now with doing this we have basically told our clients not to update these plugins as they will not function properly if they do as we edited the core of them.

Is there another way to edit plugins so that they can be updated, sort of how child theme development works?