It really depends, re-editing all of the episodes from the original film negatives would be an enormous undertaking. Every episode would pretty much need to be redone from scratch. That's even assuming the negatives are in any condition to undergo the process and won't need fixing/reproducing themselves.
Would it be that much harder than TOS? I would think the TNG film would be in better condition than TOS, being 20 years newer (assuming it was stored well). Or did they not need to re-edit the footage together for TOS?
They didn't need to edit the film for TOS since TOS was edited on film. So it was a simple matter of running the film through the process of cleaning it up and "remastering" it.
For TNG they have to go back to the editing process and put the episodes back together again from square one.