At this moment, it's not possible to change the quick name you've set for a contact.
However,
if Patchou re-inplements the Custom Names feature
(which has been left out because WLM already had a quick name feature but isn't so extended as Plus!' one), I guess he'll probably add a property for the Contact object to get/set his/her custom name.
I'm just speculating, it depends if Patchou will add this or not.