The upgrade was delayed and not completed on February 28. The upgrade is now scheduled for March 19, 2019.

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Previous Description (February 19, 2019 13:32):

All apps and flows created in PowerApps and Flow will be upgraded to a new database version on February 28 2019.

To ensure that apps and flows continue to work after the upgrade, they must be verified by February 27 2019.

Users of these services will be receiving emails with instructions on how to complete the verification process or see Microsoft's tutorial on the verification process.

For more information on the new database version, see Microsoft’s blog post about the new Common Data Service version.