According to Allan Odgaard:
I should add that (also starting with r1019) the customizations stored in ~/Library/Application Support/TextMate/Bundles are actual delta-files. Meaning that it is now possible to edit key equivalent, scope selector, name, folding patterns of a language grammar, or similar for a non-local item, and still see the other properties of this item update in the future (when the non-local version gets updated).
That's good news Allan, thanks!