Fine !
I did this upgrade yesterday (because of other issues described in a cry for help elsewhere), and ran the mysql_fix_script as advised on dev.mysql.com.
Luckily I have a dbtools manager installed, and that helped with my specific problem creating a grant (Thx, Dan, for your pointers in the 'Change Mysql password-howto').
Alls well that ends well!
But my yum hasn't figured out the update. This morning it presented me with a possibility to upgrade mysql to version 3.23.56-1.73
Apparently yum checks only version difference, not whether the installed version number is actually higher than the proposed.
How about it, Darrell ? (Not that I don't love your work! It has helped me out more times than I can count!).