The way it works for warlocks is that our dots update
BUFFS on refresh (either when you hardcast them, through applying them with Soulburn: Soul Swap or through the use of Felflame) and then keep those stats till the next refresh. With
DEBUFFS our dots (and Malefic Grasp and other spells) change dynamically from the exact moment we have the DEBUFF on target till the moment it is gone. So you're fine channeling MG at once, cause Haunt will update the damage ticks from the moment it lands (as well as the dot ticks) till the moment it ends.
EDIT: should add that Haunt is a debuff, just to be sure