When migrating to Exchagne 2010 from 2007 you can easily test if an account is migrated with the following
In server catelog set a AD attribute msExchRBACPolicyLink. That is only created in Exchagne 2010.
Then create an expression in roles NOT isEmpty(userAttr.msExchRBACPolicyLink) If not blank send user to the OWA 2010 role. Esle user gets the OWA 2007 role