This is D&D; WotC would have to diffuse criticism even if they delivered to us all free books and cake.
What type of cake cause I hate carrot cake, and what if I'm not home when the books are delivered and its raining........ .
Really though, as was said Migrations and Upgrades are typically measured in hours in the IT world.
But we do have to keep in mind the general spagetti nature of what the WOTC boards had become. Spagetti code is a programming nightmare.
As well as most of those boards were originally formed prior to the 'standard' bulletin board software available nowadays.
Since we were warned about it ahead of time I could care less, I don't spend very much time there anyways maybe once a month at most. Perhaps the changes might make me want to go there more often.
So I'll give them the benefit of the doubt, if they are migrating platforms as well as making upgrades all around, along with saving all those archives I can see how there will be a great downtime. Think about how many posts on average there are every day on there, perhaps this method allows them to fully migrate the entire database of posts.
I've done a few migrations like that before on Company Forums that were provided free to anyone that wanted not just customers. Behind the scenes this took us weeks to prepare, we are all volunteer moderators/admin (only one paid member on staff). It was a move of platforms and software, took me a better part of a week of evenings to go through all users (10k+) to update their permissions properly (We added private areas for various interests). Blogs/Galleries/New Forums/Private Forums were all added at the same time. So I can see where time consumption can go into prior to the 'flip the switch moment'.
They might be changing the web addresses as well for the direct forum links, so there could be quite a few things they actually need to wait for certain things to update after a couple of days.
