Dror,
Are you familiar with the xnmsnmpconf command?
It has several options that I have used at customer sites to
do exactly what you are asking.  This will NOT work if the workstations
are defined by a collection because the xnmsnmpconf import and export
options do not support collections. However, this is an option that I
have
used with Specific Nodes or Address Ranges.

1. xnmsnmpconf -export day_ovsnmp.conf
2. copy day_netmon.conf to night_ovsnmp.conf
3. edit day_ovsnmp.conf so that workstations are polled 1/yr
4. edit night_netmon.conf so that workstations are polled regularly
5. Use crontab entrys to call a script that executes xnmsnmpconf -import

     with the appropriate night/day file at various times of day.
    The script will also need to execute xnmsnmpconf -clearCache
     (to purge old config) and xnmsnmpconf -event
     (to notify apps that there is a new config).


======================================================
Don Davis  -  Instructor / Tivoli Certified Consultant
Visit our Web site at: http://www.apc-services.com