TeamCity 10.0.1 Update
We have prepared a 10.0.1 bugfix update based on the feedback we received from those who upgraded. The most critical issues found in 10.0 were documented in the Known Issues section of Upgrade Notes and we provided patches for those affected. All the critical issues related to upgrade are now fixed in this 10.0.1 update. There are a lot of other fixes as well.
All the bugfix 10.0.x releases use the same data format and require a license covering 10.0 release date (21 July 2016), so you can upgrade and downgrade the server within 10.0.x releases freely.