Mythtv 0.21 did a weekly backup. These backups should be saved, just in case...
Since 0.21 did precede the "official" backup script, one may do the "up-to-the-minute" backup from a user prompt as follows:
There is no space between -p and the actual password.
mysqldump -u mythtv -p[mysql-password] --extended-insert --databases mythconverg > mythdatabase.bak
Since this command omits the "--no-create-db" and "--add-drop-table" switches, the assumption is that the new, freshly installed but essentially empty Mythtv 0.24 mythconverg will have been dropped before the 0.21 "mythdatabase.bak" is restored into its place.
The restoration of the saved old database into its new home will be as follows:
Then, of course, starting the backend from a prompt will upgrade the 0.21 db schema to the appropriate point.
mysql -u mythtv -p[new-mysql-password] mythconverg < mythdatabase.bak