Well, of course there's a lot of things to add / edit, but there's also a lot of thing that stay mostly inchanged.
For instance, if you take the "Gameplay elements" of the DF wiki main page :
Activity zones : Except for burrows, the whole fishing or meeting areas won't change at all I guess.
Farming : Won't change. The plants raws will change a bit probably, but that's a minor issue.
Building : Won't change much, also.
Combat : Of course, almost the whole category will need to be rewritten.
Creatures : Except from the raws, most creature will keep their current behavior, I guess. Will need some testing probably.
Designations : Like activity zone, there won't be a lot of change except for Burrows.
Military : Same as combat, the new squads will need a total revamping of the page.
Nobles : Will probably need some updates.
Rooms : Again, there will be burrow-related updates. Also, hospitals.
Skills : New skills, skill rusting and the other skill/attributes changes will need to be added.
Stockpiles : I don't think there will be a lot of change on that side.
Workshops : There might be new buildings, but I doubt it. Minor changes in some building operation (smelters / smith workshop are the more likely) may happens.
Economics : Hm, the trade process won't change much, but items production and value may change quite a bit.
Mining : The whole cavern and burrow things will probably have to be mentioned.
Location : While the above ground won't change, the underground will have to be updated.
Fun : To update, indeed.
In addition, I guess we'll need to add pages for each new features, change pages like "fortress design", inflate the "venom" or "health care" pages (or turn them into categories), and also something for the new descriptions.