Modifying part of room reservation for discount

[KCanuck]

I have a room booked Oct 6-12. The rumour is that when room discounts are announced they will be up to Oct 7. If my resort room category has availability for a discount (fingers crossed), will I be able to modify the nights of Oct 6 and 7 online? I dread calling on the first day of a promotion! I suppose I could book those two nights online and then cancel those from my original rack rate reservation? Thanks for any input.