As a rule of thumb, how long does it take to make changes like that to existing maps? Taking the mounts into old maps has been hilarious, sometimes it's hit and miss though. I keep dying in Verdant Brink because I keep trying to fly to the flax farm from Noble's Folly. I build up a ton of speed, sanic my way over there, dodge vines and low hanging rocks, lightly graze the killwall that is VB's map boundary, then watch my corpse gracefully descend into the depths. I also got up quite high in the Silverwastes and flew to the airships overhead only to find that they have no collision and you fly straight through the texture.
I was one of the players extremely leery at the prospect of mounts in Guild Wars 2. I, somewhat cynically, thought it was just going to be WoW pandering but the way they've been implemented has been a ton of fun and it's truly affected gameplay more than the speedboost and cosmetic goldsink I was expecting. You've all done a fantastic job.
It usually took us a day or two to modify the actual borders of each map and then any additional fixes for falling through props and so on was additional time. It usually didn't take long to make collision changes, but it was hard to catch them all.