Technically, as the 3750 lacks a microphone, you cannot convert it to 3490 using a sw mod solely. Lacking BT & ASR functions it would be a 3450. The 3760 has a mike and can therefore convert to 3490. It is possible to solder in a mike on the 3750 board though, a somewhat delicate procedure and risky for the ham-fisted [i.e. yours truly]: [Only registered and activated users can see links. Click Here To Register...]. So if you've added mike you've converted to 3490, or if not, you've converted to 3450.