My host doesn't have yum, but I wound up putting MySQL 5.1 out there - ready to instal. Your reply makes it seem that I just do this upgrade, but I want to be sure which of several possible paths to follow so Plesk continues to function normally. I know there are *some* restrictions, because the PLESK startup script S85psa assumes there's a mysql user named "admin", that the socket is in "var/lib", etc. But it also uses a variable $MYSQL_BIN_D, which suggests that MySQL can in fact be anywhere. So specifically, which path should I take:
Path 1: Install on top of the existing MySQL (/usr) and upgrade the psa database.
Path 2: Install in a completely separate place (/usr/local) and let PLESK continue to use the version it's using - tempting, but they can't both use port 3306.