I posted here about the differences between the early and later MK2 serial cables. You could quite easily rewire it yourself, but you would need a new serial connector and cable as the existing one doesn't have enough wires in it.

(list 6284, Mk1 S/N 00299 4GB blue [for sale]. Mk2 S/N 080000094 26GB blue)
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(list 6284, Mk1 S/N 00299 4GB blue [sold]. Mk2 S/N 080000094 20GB blue)