You can compare the cnaged file with the orginal file to see what changes are made.
My files are too heavily modified to compare to yours to see what has been changed, so this isn't really a solution.

In order to do this, I would have to first download an old version of the files. I can't do this, because my download link now downloads version 2.5, so it's senseless to download that to compare.

Surely you know what you changed. What is the harm in putting out a little documentation to say style.css had x, y and z changed, tpl files had a and b changed?

I dont think this is necessary with most files. But when you start changing template files and style sheets, it would only be a basic courtesy to let people know what the changes were, since these are the files that most people would have changed when moving away from the look of the "out of the box" template. If you had some kind of basic documentation, this upgrade would take an hour. Without it, it will take me over a week to (unnecessarily) remodify all my template files, and I suspect it would be all for nothing since I've already fixed most of the errors you had in those files.