Yeah, but this post is about removing alt constraint that exists below accln alt (after takeoff without head down work)!!
Okay, sorry for being pedantic, but can we be precise with language here? Is it possible to have an altitude constraint below acceleration altitude? The point about acceleration altitude is that you have to maintain a minimum gradient to that point - so an altitude constraint below there would undermine the basis of the acceleration altitude, no? Can you point me to a place where there is an altitude constraint below acceleration altitude? Or is that not what you meant?
Based on the question: "How do you remove an altitude constraint in the flightplan whilst you are still below acceleration altitude?" my answer is still, "Don't bother - wait till acceleration altitude and then take open climb - but that's not an SOP, just my preference." If non-essential ECAM actions can be delayed till the aircraft is clean, then I think non-essential FMGC work can be as well.