Changing Domain Controller for Exchange 2013

Folks

I have an Exchange 2013 running on Server 2012.

Old DC on Server 2008.

I want to decommission the 2008 server. I have build a 2012 DC and for the life of can not work out how to change the DC that my exchange box uses????

Using Set-ADServerSettings cmdlet only seems to change the server for that current session?? reboot and back to the old DC...

When I use the Se-ExchangeServer cmdlet, I get domain controller cant be found. I have set the execution policy on the dc to unrestricted and still Domain controlleer cant be found..

New dc is a GC..

Any ideas would be good.

-graham