Merging ressies is not as easy as some of you seem to think. You can't just snap your fingers and have it magically happen. But, most of you have not worked at a hotel so you do not the actual steps for any part of a process.
Merging multiple reservations into a single requires cancelling the additional reservations, opening up the first one and adding the additional nights, if needed, override the rates for each night to manually put in the rates from the individual nights' reservations or add another rate plan to the existing reservation for those nights. Depending on the individual hotel and it's revenue management policy, the Front Desk may not be permitted to mess with this.
As for the idea that this has to be done to keep the same room, not true. There is a wonderful procedure in Lightspeed called "link". The properly allowed people at the hotel are able to easily link multiple reservations to ensure 1)you get the same room throughout the stay, 2) check in once physically - the rest is done everyday by the Front Desk, 3) make keys good for the entire stay (if the key system permits overriding departure dates, 4)at some hotels, the credit card information is automatically added to each reservation each day.

Where I worked, we never merged, we always linked. Now, if someone asked to extend a stay, it worked the same way - depending on the rate plan, it could not be done by adding a night to a reservation. A new ressie had to be made and then linked to the original.