I'm assuming that your running AP SSO. If so, then you just have to remove the old and configure the new one with the basic setup to get it on the network. Then you just need to make sure that the existing is set to primary, which it should be and then make sure the mobility mac address is identical. The unit id will be different, but the mobility mac is what you need to make sure is identical. Then you can failover to the new if that will be your primary.