DEP - How to enable device auto-assignment to an existing MDM server

When you add a new server to the DEP portal you have an option to “Automatically assign new devices” to the MDM server.

However there does not seem to be an obvious option present for existing servers, after playing around with the DEP portal further we’ve realised that the icon on the left of the server is actually a button and if pressed will automatically assign all future devices to the said MDM server.