Pulling the ALT knob and going into Open Climb cancels your acceleration altitude constraint, which maybe for either terrain clearance or for noise. If you want to keep it, you need to control the speed manually. Either by pulling the speed knob and adjusting the speed to your previous SRS speed, or if you can anticipate this happening in the planning phase, put a suitable manual speed in the climb phase.
In either case, you lose SRS protection, so another way is to dial up the new altitude/FL and go into open climb by pulling it once you have accelerated. If you haven't already gone into ALT* and there is no alt constraint, then it will accept it as a new climb altititude/FL and climb accordingly. If there is, you can get always the other pilot to cancel the constraint.
I wouldn't personally use VS as it won't maintain a speed reference.