Preferred form for making modifications

You are also obliged to make note of any modifications you have made (no requirements for changelogs though) and when you made them.

"You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change."

The best way to do this *is* typically to provide a changelog entry, but if you just distribute patches that should be enough; after all patches by their very nature show what files are modified, and assuming that the patch was created when the modification was made, it'll also have the time of change in the patch header.