We had a 6 Terabyte array and all the original HDV footage that was deemed needed for the show was digitized onto that - so if we ever need to make a change to the graded footage the original footage was always still accessible, it just was a lot smaller than the Prores transcode would have been so we could keep it accessible. Occassionally we did have to go back to tape, generally because of when reviewing and edit for technical specs we would pick up HDV artifacting we hadn't noticed while editing, and actually needed to completely remove the shot or change the part of the shot that was used.

When we needed to make a change like this after the footage had been graded, it was really just exporting a stand alone Prores quicktime of only the new footage to be dropped in with handles, and bringing this into a Color project where we could Grade that to match in color, and then bring just the new shots back into Final Cut and replace the problem shot in the graded timeline.

Got slightly trickier if graphics/transitions were involved sometimes, might have to change the shot before and after the edited shot and regrade those to accomodate a change in transition as well, but that's easily achieved by saving the grades in Color as a stand alone grade then applying it back to the footage in the new project.

Baseline keys etc, don't go to the grade obviously, they all just stay as stand alone elements on a locked final timeline.