From Nexcess:

 

During this migration there will be a brief interruption in service(s) associated with your server. Our migration team will be migrating servers between the hours of 12:00AM and 6:00AM Eastern Time on  September 30th. While we have scheduled a six hour window for the migration, your service will not be interrupted for more than two hours. Our migration coordinators will be monitoring your service before, during, and after your migration.

Your IP addresses will not be changing nor will you be issued new hardware during this migration; rather we are physically moving your service to the new facility.  No changes are required whatsoever on your part.