Sorry I misunderstood your request and thanks for clarifying.
I would create a scheduled job in NCM.
We are running NCM 7.1.1 –
From Network Configuration Manager Schedule > Create New Job > Upload Changes to Devices > the time and frequency the job needs to run.It will open a new window of the newly created job to be modified. In Nodes tab change “All nodes in the database” to “Specify a Selection Criteria” and Add a Simple Condition where Discovered Properties is Vendor and is equal to Juniper.
In Upload Config tab – if these were Cisco boxes I would’ve chosen a “blank Config” and type in all the changes that I need to be applied. If you do not know the commands to be applied you can “Select Config Changes from Database” and choose a configuration of one of the Juniper boxes. Find the line that has SNMP config ( I assume you should have this configuration line as you already have the nodes in NCM and downloaded configuration) , change the SNMP setting to reflect your new test NCM server settings and remove the rest of config from the upload config window.
You can schedule it to run once at a specific time. I would probably test it on one node first (least important node) before applying to a bunch of nodes. Make sure you have a latest configuration backup and the configuration restore/recovery steps are known.
I hope this is what you were looking for.