I think listing 4.1 needs a revdep-rebuild to get back to the old version of mysql. At least I did.
The link in your post is for the kernel upgrade guide. Code listing 4.1 in that upgrade guide is entirely irrelevant. You sure you're talking about the right doc? The kernel has nothing to do with mysql.
Ah, you mean the mysql upgrade guide. CCing one of the authors for comment. CHTEKK on irc thought a revdep-rebuild couldn't hurt, but I'd rather not include unnecessary steps. <@CHTEKK> well... if you upgraded, then downgraded again, without recompiling other stuff <@CHTEKK> you theoretically don't have to
listing 3.3 does have you recompile everything else, right after emerging mysql. I went completely through the upgrade, including emerge, revdep-rebuild, and restoring the databases. After everything was loaded was when I discovered the problem that required me to downgrade.
Downgrading
Ahem. Previous comment was cut off. :) Anyway, despite thousands of users who have not reported any failures with this guide, we added a note about possibly having to run revdep-rebuild after downgrading. Just in case. Fixed in CVS.