VZW EVDO Rev. A updgradeable?

[Unregistered]

What does Rev.A upgradeable mean? Some of their phones already have Rev.A, so does that mean it's a hardware thing or a software thing? For example, I see that the Curve is not upgradeable. The upcoming HTC Diamond is updgradeable - will VZW provide a software upgrade to make it Rev.A?

If that is the case, why not make it Rev.A right of the bat?!