The AltMill uses closed loop steppers rated for 600IPM with external motor drivers plugged into the SLB-EXT controller, whereas the LongMill MK2 uses open loop steppers with integrated drivers directly soldered on the SLB controller. Because of these major differences, you would need to purchase the SLB-EXT controller (and 8mm to 8mm couplers) in addition to the closed loop steppers to make this modification.
If you are looking to make the modification, please be aware that our customer support team may not be able to provide troubleshooting and best practices on this matter, as we have not tested the LongMill with this. However, feel free to send us progress updates!