A couple of questions:

1)  We recently changed SNMP configurations on a lot of devices which were already discovered.   Is there a way to bulk change the SNMP community used by a group of devices?

2)  For "unknown" devices, now discoverable via SNMP, is there a way to force immediate discovery of the device and get it added into the database?   

 If you look on the Settings tab under "SNMP Collector," there is a button underneath the table of credentials that will effectively reset all of the learned/programmed SNMP strings for every device.  So NetMRI will take another swing at the previously unknown devices, and even those devices that have been manually set (under Device Viewer) will be guessed again.

 Thanks, I had seen that option but was looking for something that would reset on a group of devices and not across the board.

 Unfortunately today there isn't an easy way to this.  You can do it a single device at a time, or globally.  In my mind (and past practice), I say do the global reset.  (1) You will automatically (attempt to) pick up more from the unknown table.  (2) You will ensure that all devices that have changed will automatically be re-configured, as opposed to a grouping or other mechanism where some might fall through the cracks.  (3) You validate that all of the devices you expected to be able to access via SNMP are in fact accessible (still?).  

In other words, I agree it perhaps is not the ideal answer for you, but from my perspective, I see enough benefit in this approach to go for the larger, 'overkill' answer.  I guess the only caveat would be in an environment where I have devices (like UPS's) that are highly intolerant to multiple SNMP connection attempts.