Upgrades should never take a week. Clone the server offline, perform the upgrade. Take notes. Rinse and repeat. Eventually, you will have a "playbook" to perform an upgrade, know all the steps, know exactly how long it will take, and finally, you will know if something isn't going right.
With a scale of hundreds of thousands, I have one chance to get it right. I never want the first time to be that one chance.
MGO was switching servers. that requires a full data backup, then move everything over and reload the data. its not a over night thing and they probably wanted to make enough time alloted for the move.
There are ways to do incremental transfers, even down to the transaction level.