If you go in and blam an entire page, no doubt the original authors will cry foul; it probably took them a fair amount of time, even if what they wrote wasn't particularly good. It's also fairly rude without at least giving them a chance to improve sections.
Incremental editing will also give you a far stronger case when they get arsey, particularly if your inclusions are better than what was there already.
The key is trying to work with the grumpy people and reassure them that you're not trying to take their article away from them.