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Before upgrading, please make sure the maintenance period of your licenses is not yet elapsed (use Administration | Licenses TeamCity server web UI page to list your license keys). The licenses are valid only for the versions of TeamCity with the effective release date within the maintenance period. See the effective Check the effective release date on this pagethe releases list.
Typically all the bugfix updates (indicated by changes in the Z part of the X.Y.Z TeamCity version) use the same effective release date (that of the major/minor release).
If not all the licenses cover the target version release date, consider renewing the licenses before the upgrade (you can replace the old license keys with the renewed ones even before the upgrade).
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