[KaVir]

I would have to judge it on a case-by-case basis. In extreme situations I might simply scrap the content (I recently dumped an entire gameworld and replaced it with a new one, but the original had only ever been intended as a temporary thing anyway). Generally though, I'd just tweak things that didn't properly mesh, and update the writing if I felt it wasn't up to scratch.

If the original area fitted thematically but the writing needed a complete overhaul, I might be tempted to tie that change into the storyline - i.e., rewrite the area as if some great event had just occurred, then run that event as a major one-off quest, and swap the areas afterwards. This can make the game world feel a bit less static.