We plan on upgrading TeamCity to 2018.1 later this week.
In preparation, we may need to take it offline briefly over the next day or two. We will send out a warning an hour before any such outage. They should last no more than 15 minutes; in-progress builds will be unaffected and the server should pick up right where it left off once it comes back up.

The upgrade itself may take the better part of a day. This thread will be updated at least 24 hours before we start the upgrade process.