Build Agent license grants you the right to:
A new Build Agent license includes Upgrade Subscription qualifying for free upgrades to any new TeamCity released during the 1-year period following the license purchase. To continue upgrading your build agents after that period, you will need to renew Build Agent Upgrade Subscription for $149 per build agent/year. If you use TeamCity Enterprise Server, you will also need to renew your Enterprise Server Upgrade Subscription to ensure compatibility of your build agents with your Enterprise Server.
Your new Upgrade Subscription term will commence from the expiration of your previous Upgrade Subscription term regardless of the actual subscription renewal date. Obtaining the new version of TeamCity under Upgrade Subscription automatically terminates your rights to use previous versions of TeamCity.
For more details, please see the TeamCity License Agreement.