Do has demazter said and decomission your 2007 CAS moving everything to 2010 CAS server. To coexist with the 2003 OWA you need to configure the legacy url in exchange 2010 CAS. To do that run the following command:
Set-OWAVirtualDirectory
exchange 2010 CAS will receive the request for OWA and if the mailbox is from 2003 it will redirect the user to the outside world and it will go in again to the 2003 url. for example if you have ISA publishing OWA you will need publishing rules for both 2010 and 2003.
Note that to create a publish rule for 2010 on ISA use the 2007 template and add the /ecp/* on the paths tab.
Also note that forms based authentication must be enabled on exchange 2003 for you to get a single sign on scenario.
you also need to change point all external urls to the 2010 CAS and configure 2010 CAS external url atributes on the server.
see this link for help:
http://technet.microsoft.c
it's very usefull
hope it helps
