If you line select the right hand MCDU key for the fix with both the speed and altitude constraints, the VERT REV page for that fix will open. You then have the option to clear only the altitude constraint, leaving the speed constraint in place. I would have the PM accomplish this task. If you are the PF and you feel task saturated, engage the A/P. Then you should be able to monitor the PM's MCDU inputs, or simply enter them yourself. Unless you are extremely light and at TOGA Thrust, you should have plenty of time to clear the constraint prior to ALT CSTR*.
Best regards,
The Kid