Update modified files

Over the time I had to modify many files in order to fix some bugs or to make adjustment to the theme. Because of the system used to update (replacing theme files) it is impossible to update the theme with the lastet fixes from the devs and in the same time to keep my changes.

Is there any way I can change only the files you modified, without affecting my changes? (this means I have to knoe EXACTLY which files where changed).

Is there a way to check, code based, the changes for each file you made? So I can decide myself if I want the change or not