Quack,
The migration has already started but we took care of server with no or very low user impact first. Very soon we'll be migrating our two main servers and there will be some disruption. DNS, mail deliveries, IRC and few other things should not be impacted much since they are available on multiple nodes but websites (including the webmail and stuffcloud) and a few other services will have a short downtime. Redmine instances will also need more attention and may be down a little bit.
The start/end of each migration, reboot… will be announced on IRC.
\_o<