I am assuming you mean upgrade a regular motherboard to take DDR ram. It is a different length then regular ram, and won't fit in the same slot. If it can already take slower versions of DDR, then its possable that they could do it, but don't want to for cost reasons, or is physicaly impossable. It depends how the motherboard controls it. They may have never planned for it in the furture, and therefore all the nessicary chips are unflashable.
AnthraX101