I will try to answer this with the experience from major upgrades before.
A distributed master-slave setup must be upgraded as one big upgrade.
What i have done before is the following and for 2.0 it would be the same.
First step forbid any changes on the master WATO. Upgrade all slaves one after another.
Livestatus should also work without problem between the versions. So no need to shutdown the complete system. As the last system i upgrade the master and allow changes again.
Another way is to upgrade the master first and do all slaves then. Here you need to pay attention that changes are only activated on the already upgraded slaves. If you are the only main admin on the system you can also do this way. What can happen here (this was before 1.4 to 1.5 i think) is that upgraded main site complains a little bit about missing livestatus features in the old slaves. Before there where changes in livestatus tables.