I told him to back up mysql.so and lib_mysql.so files and warned him of what issues he might run into based on my "Upgrade to MySQL 5 post."
After he tried, he ran into a few additional problems. These problems, their fix (and his comments) are mentioned below for reference.
(in mysql.log) [ERROR] Fatal error: Can't change to run as user 'mysql' ; Please check that the user exists!
There was no system user 'mysql', which was apparently expected by the new version. I had to add that user ("useradd mysql").
(in mysql.log) [ERROR] Can't start server : Bind on unix socket: Permission denied
All of the permissions on /var/lib/mysql were set to 27:27, so I had to change those permissions ("chown mysql:mysql /var/lib/mysql -R")
for some reason the system didn't have a "root" mysql user, which is expected by mysql_fix_privilege_tables.
The script is easily edited accordingly.
(in error_log) PHP Warning: Unknown(): Unable to load dynamic library '/usr/lib/php4/mysql.so' - libmysqlclient.so.10: cannot open shared object file: No such file or directory in Unknown on line 0
I had forgotten to check this (per your instructions). Thank goodness I did copy that file (libmysql.so.10) before installing (THANKS for informing me about this in advance). I moved it back to /usr/lib/mysql then restarted apache, which did the trick.
Let me know if you ran into an issue while upgrading that I haven't documented here.
Convincing a DBA to upgrade
Upgrading to MySQL 5
Transferring databases from MySQL 3.XX to MySQL 5
MySQL 5 Tuning and Optimization
mysql_connect(): Call to undefined function