Often single night bookings are cheaper than multi-night bookings. That is why it is often recommended on the forum that you always check the price of individual nights for a proposed multi-night stay. The hotel almost always just merges the bookings together at checkin and you stay in the same room. The only time I have had to change rooms is when they have offered an upgrade to a suite that is only available for the first night or something similar.
See these threads (among others):
nightly rate vs rate for two-night stays
Rate change during stay
Back to back nights at different rates?