The Server Move Notification Plugin adds a simple statement to the top of the admin panel, alerting the contributors to your site that any edits they make will be lost due to the site undergoing a move to another server.

You can use the plugin either as a regular plugin, or as an mu-plugin (useful for preventing other administrators from turning it off).

Download Serve Move Notification

If you need to change the text in the plugin, simply edit the plugin directly. Since this is intended as a once off thing during a server move, you don’t need to worry about updates, breaking compatibility etc. Just make sure it works for the few hours you need it, then delete it :)