Thanks for the reply.
The Quick Note, at least in the prior version, is definitely saved as it persists after closing the app and restarting the computer.
As for how many warnings, I would expect one, which said that a Quick Note, unlike the rest of the data, will not be transferred to the new version. As far as I knew, everything was saved and closed before I updated.
As I mentioned, I have a backup, so rolling back the update might be a solution if there are instructions somewhere how to do that.