[CentOS] Q about CentOS-4

I have a client with a server on CentOS-4 and I'm looking to upgrade to
latest horde/imp/etc.

clearly php 4.3.9-xx is not up to the task but I see that upgrades for
php & mysql are in CentOS-Plus

I see that CentOS-4 projects to get security updates until 3/2012 which
probably is the usefulness of the hardware itself.

So my questions...

- Will CentOS-Plus maintain the php/mysql packages until then?

- The upgrade to mysql-5...do I have to nuke the db and reload from
backup or is there a possibility that the upgrade and subsequent restart
of mysql will update the database stuff itself (of course I'm gonna do a
complete dump of mysql data before attempting)?

Craig