Combat Declaration and Spells

[Druknal]

Presume I declare an attacker without flying, and my opponent declare a blocker without flying. If I instant speed give my creature flying (because of it being combat step) will it deal combat damage to the player, or be pointless because a blocker was already declared and damage is still delt between the two?
My guess is that I would have to cast it as I declare my attackers, before the defenders step. Yes?