Migrating from BSD 3.1 to Linux?

Slain asks: "I've searched the internet looking for information about migrating from BSD to Linux. I have not found anything on the subject. Is it something that is just not done? What if you wanted to switch for cost reasons? Is it not done because you are already running a flavor of UNIX so why switch? I have a BSD box which is used as the primary internet mail server for the company. It has about 700+ users and grows about 20+ a week. Why do we want to migrate? The reason are as follows: newer hardware, user maintenance and we want to reduce services to the users (eg. make them POP only accounts). Anyone out there have any suggestions?