If a customer installs a Professional license will they be able to upgrade later to Enterprise
Yes through deployment manager, for more see following link , if you have MBS partner access to download pricing and upgrade options for CRM 4.0
http://blogs.msdn.com/davidlem/archive/2008/01/09/new-crm-4-content-good-stuff.aspx
Best Regards,
Imran
http://microsoftcrm3.blogspot.com