Changing the mysql.proc table directly after upgrade
The mysql_upgrade utility has an --upgrade-system-tables (only) option.

I tested it on a local machine that had already been done, but judging from the output and the documentation, it seems to be what you're looking for.

Of course, be sure you use the version of mysql_upgrade that exactly matches whatever version they have upgraded you to (e.g., 5.5.30).

sqlbot@dev:/usr/local/mysql$ bin/mysql_upgrade --upgrade-system-tables
Looking for 'mysql' as: bin/mysql
The --upgrade-system-tables option was used, databases won't be touched.
This installation of MySQL is already upgraded to 5.5.30, use --force if you still need to run mysql_upgrade
sqlbot@dev:/usr/local/mysql$