DNS server names suddenly changed

Hello all. We have many sites here, and recently noticed that the DNS server names we normally receive changed recently. Everything works fine, but are wondering why that might be and if we’ll have to change the sites we already have up.

Also, has this happened to others?

Nameservers typically are account-bound but AFAIK not necessarily account-bound. I wouldnt expect the addresses to have changed for existing domains, but you can certainly check that on the DNS screen of each domain.