Today the only two smartphones running WM2003 are SPV E200 and Everlink
(part of the development kit) after free upgrade. The rest does not have an
officially released upgrade yet.
That's more of a hardware question than software. If the hardware provider
sets things up to be upgradeable, there's certainly nothing that the OS does
to prevent that.
All Smartphone devices *are* physically upgradeable but it is up to the
mobile operators like Verizon to make upgrades available. Various business
factors determine if/when/how they'll do that. So far, nothing has been
announced.