Migrating Multisite To New Server With New IP's

We are getting ready to upgrade our servers. Managed services has everything migrated, but we had to acquire new IP's. I am looking for the smoothest way to migrate multisite.

Here are the variables:

• Custom NS that the majority of customer use currently assigned to legacy server IP
• Other customers point A-Record Only to legacy IP
• We want to decommission the old server

Our thoughts....

We were going to first change the IP on the NS to point to the new IP's, then we are going to send out a memo to our a-record clients about the update and ask them to update their IP to resolve to the new server.

We do this all the time with non-multisite websites, but where we are a little hung up is: "If we change the IP on our primary multisite network NS's, will our clients that use a-records still resolve to the old server until they update their records?"

If the sites using a-records won't resolve properly as the NS is propagating to the new IP, would you then make NS3 and NS4 to point to previous server's IP address to mitigate the issue or would that further confuse things?

The wildcard for us is understanding how the a-record sites will function during the IP change. We feel comfortable the sites that changed their NS will flow naturally.

Thanks for your advice.