I've got a 4 night confirmed award reservation for a Thurs-Mon, cat 5 hotel, linked to a 75K 3 night redemption and also a 1 night cat 5 visa certificate.

The hotel is now completely sold out for those nights.

I'd now like to add the Wednesday to the reservation. Wednesday night does show award rooms available. I could just book it as a standalone 25K night, bringing my total cost to 100K + the visa cert. But since that would make it a 5 night stay, I'd prefer to pay for the whole stay with a 100K 5th-night-free redemption (thus saving my visa free night for another trip).

Is that typically possible? Or would this require re-booking the whole thing, clearly not an option since the hotel is sold out over the weekend. I'm hesitant to even call for fear they mess up the existing reservation; it's not just the hotel that's sold out but pretty much the whole city.