Benjammin, you can't replace the Mk2 knob with a Mk2a keyed knob. I think you'd have to replace the whole display board if you did, or at least do some very tricky desoldering and resoldering.

Instead, do as was suggested and contact support. The FAQ entry on this topic is here.

They send you a different kind of knob that's better designed to grip the round shaft, but they don't replace the shaft itself.