It all depends on how much changed
In this case there are changes to spells (which in turn means abilities/items/other effects) along with a task revamp.
Basically the developers not only have to identify and reconnect this to that, so MQ works like it did before the patch, but also any changes made that may require additional coding, and how best to go about it... and of course test it. As this is a patch plus an expansion release, there could be plenty that were not covered in the beta/test. SOE/DBG are bad about introducing changes only at patch and not test.
Your best bet is to just plan on it being down for a few days and just check the boards now and again. There ought to be a post saying to run the patcher coming along shortly =)
And if you can manage it, toss the developers some beer money. Remember this is all voluntary work, and it takes away from not only their game play but also time they could be doing other things.