I am in the process of migrating a web server which has a few thousand small sites, and does its own DNS. Each site has a hostname in the form of "customer.ourcompany.com" and some also have "www.customersdomainname.com".
When we do the migration, the IP is going to change, so we need to update all DNS entries for all domains. Because this machine is also the authority for ourcompany.com, the IP for ns1.ourcompany.com must also be changed.
This is the problem. For all of the client domains, we need to make sure that any glue records will contain the correct IP.
Are glue records always used by registrars, even if they are not technically needed for a domain? We migrated another webserver once, and I needed to log in to the registrar's site (GoDaddy) and update EVERY namerserver entry by simply swapping ns1 for ns2 and vice-versa. This forced GoDaddy to lookup the new IPs for nameservers, and store them as glue records. I am afraid of having to do this again, but with 2000 domains, not all at the same registrar.