Fiirstly is it NYE you are after?
actually which hotel is it?
and NO you cannot book for one period then change to another expecting to be accommodated and certainly not for the week long price.
I am booked in melbourne OZ during the grand prix.
the price for 5 days is 250 inc breakky parking etc the price for 4 is 395 bare room. so I took the 5 days as it is very much cheaper. ^
I cannot then expect them to cancel one night and give me the 250 rate!