There is a listing of all the FMS updates at
The 737 Technical Site
The bit you are looking for is in update 6:
Update 6.0 - 10 Jun 1992 - Ref 737-SL-34-93
1. Speed Intervention
Allows the flight crew to override the FMC target speed without disengaging vertical navigation (VNAV) through the use of the mode control panel (MCP) speed intervention on/off pushbutton, the MCP indicated airspeed/mach select pushbutton and the MCP speed select knob.
2. Altitude Intervention
Using the MCP altitude select knob and the MCP altitude intervention pushbutton, the flight crew has the capability to:
a. Delete the next altitude constraint while in climb or descent.
b. Increase the cruise altitude while in climb or cruise.
c. Initiate either a step climb or a transition to descent phase while in cruise phase.
Probably most airlines do not think it worth the cost for the benefit gained - just make the pilots work a bit harder and make their changes using the CDU keypad.
