In the upgrade doc it says:

To configure 5.x media servers for compatibility with 6.0 environments
NetBackup 6.0 provides backlevel support for media servers running
NetBackup 5.0MP4 (or later) or 5.1. This enables you to upgrade some
or all of your media servers to NetBackup 6.0 at a later time.
However, you must modify the configuration slightly so version 5.x
media servers work in the NetBackup 6.0 environment.
1. Run the nbpushdata -modify_5x_hosts command on the NetBackup 6.0
master servers that administer the media servers that you want to
remain at version 5.x.
Note The command option is literally modify_5x_hosts, and uses the
character x rather than a version number. Do not type either
modify_50_hosts or modify_51_hosts.
/usr/openv/netbackup/bin/nbpushdata -modify_5x_hosts
60 NetBackup Installation Guide for UNIX and Linux
Upgrading NetBackup on UNIX Servers and Clients
2. Repeat step 1 on any additional master servers that administer
media server(s) you are not upgrading to version 6.0. You only need to
run this command once on each applicable master server.

So we just run: nbpushdata -modify_5x_hosts hostname1
                       nbpushdata -modify_5x_hosts hostname2

(after the nbpushdata -add)

For each NetBackup media server that still resides on NB 5.x?

Justin.