Assume you've built your new Exchange 2007 CAS server and applied the latest update rollup. All is well. Then you decide you need an additional Hub Transport server in your organisation so you add the Hub Transport role to the server currently running the CAS role. The question is: do you need to re-apply the update rollup? In fact, the question is really whether you need to re-apply the update rollup after adding any additional Exchange role to a server?
The answer is apparently 'no' as everything should be just fine. Fingers crossed, then.