From the Microsoft Service Provider web site:
Are you ready to migrate from HMC 3.5 to the new version of HMC? Take advantage of the Exchange Server 2007 enhanced enterprise-class e-mail, calendaring, contacts and the real-time collaboration available with Microsoft Windows SharePoint Services version 3.0. The HMC 4.0 Migration kit provides the tools and technical details to enable service providers to move to HMC 4.0 without impacting their end-user services.
So if you’re in for a challenge I would like to give a free advice 🙂
If you’re choice is the ‘Coexistence Migration’ and you compare the migration documentation against the HMC 4.0 installation guide be sure to use the Exchange 2007 installation wizard instead of the command line options. Otherwise I’m pretty sure that you end up without your existing Exchange 2003 SMTP Connector 🙂