You could stay with the Version you currently have, and use vmware importer to clone the 2k3 server as a virtual machine to the new box.
If you have a BES SBS and not a BPS you can upgrade to 4.1.6 by downloading the upgrades from rim at no cost.
If you later want to upgrade to BES V5, buy an upgrade key, and, install as an upgrade over the existing 4.1.x Server.
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