We are migrating from Exchange 2003 to Exchange 2007, we have Client Access Licenses (CALs) for Exchange 2003. Can we use these CALs for Exchange 2007?

That is really a question for your software vendor that sells you licenses like CDW. However as far as I know the only time an older version of a MSFT product CAL is upgradeable to the newer CAL is when you have a software assurance agreement in place (which more or less takes care of that sort of thing) or you buy ugprade CALs at a discounted price. Keep in mind that there are two server versions of Exchange (Enterprise and Standard) and two client CAL versions (Enterprise and Standard). The CAL versions refer to features in Exchange you want to use, and the server version refer to capabilities of the server. They aren't linked directly to each other which is to say you could have a Standard Edition server with Enterprise client CALs, or an Enterprise Edition server with Standard client CALs. One customer I just helped upgrade to 2007 deployed Enterprise Edition servers for their mailbox servers, and Standard Edition servers for their CAS/Hub Transport servers. They then bought a mix of Standard and Enterprise client CALs as they only needed some of the more advanced features of Exchange for only part of their staff. Good luck!

Thanks. Much appreciated.