Back when I was doing Tech Support for a webhost based out of the UK with servers in the US (I was the US point of contact) we had to move our servers from a center in Chicago (Voxel) to one in California (Hurricane Electric), we set the TTL for DNS queries ultra low so the propagation on domains was easier on the clients; but we also emailed them all like a month prior that we where migrating them to new servers (then again the servers we migrated them to where far better machines as well).
