You can change the security level from Level 1 to Level 2 by editing the /etc/inetd.conf file on each system, and adding the -S 2 option to the sadmind entry. If you do this, make sure that the servers in the domain are set up to use DES security.
You do not need to maintain the same level of security on all systems in the network. You can run some systems, such as file servers requiring strict security, at security Level 2, while running other systems at the default Level 1 security.
See the description of how to set up security for NIS+ in NIS+ and FNS Administration Guide.