Exchange 2013 DR CAS Servers Standby

Hi

I currently have 2 CAS servers in the Primary DC and 2 in the DR site.

Each site has its own Load balance.

What I want to do in the case of a DR situation is manually direct all my clients to the CAS servers in the DR site with all URL's remaining the same for OWA and autodisocver etc..

At the same time the DR site should not accept any connections from the users while the primary site is up. The DR is basically standing by.

What is the best way to implement this design? and what would i need to do to switch to CAS servers in DR?

Thanks