Currently we have a process for migrating a single config server.

We should document a process for migrating three config servers at once via change of CNAME. Say in the event of changing instance sizes.

This process can potentially cause people problems if they clone the config servers. Eg, Old Hosts A, B and C. New hosts A1, B1, C1

Shutdown A,
Clone data from A to A1, B1 and C1
Change CNAME for A to A1
Start A1

You now have all three config servers up temporarily which can lead to writes being on B and C, but not on B1 and C1

As this is something people may wish to do we should document a working process for them.