No, you can't fit a keyed control (ie mk2a's control) to a mk2. This is a blessing in disguise, as the mk2a's ALPS controls are hard to get off the shelf, whereas the mk2's rotary encoders (Meggitt) are available off the shelf from farnell.

Swapping the control isn't hard, but does involve some solder braid and delicate work. Units under warranty should be returned to empeg for this.

Hugo