Decommission Exchange 2003 after 2010 install

I'm working in a Windows Server 2003 Active Directory Domain.  I brought in a 2008 server with exchange 2010 and brought it up along side the exchange 2003 server.  Both are playing nicely.  During the exchange 2010 install, my exchange 2010 found the exchange 2003 server and installed the mailflow needed to make the two play well together.  After the install, I moved all of the 2003 mailboxes to the 2010 database and now, I no longer use the Exchange 2003 server for anything.  My firewall sends all port 25 traffic to my 2003 box still and I'm about to change that.  

Before I do, before I attempt to take the exchange 2003 box offline, I want instructions on how to do that, what to watch for and what to do so that I do not mess anything up.  

Can I simply change my firewall and send all 25 traffic to the new box and I'm good to go or is everything still going thru the 2003 box?  What steps do I take the get the 2003 box out of the loop and decommissioned?

Thanks

Cliff