The cruise lines are under no obligation to give you a higher category cabin at a lower price...just like they don't do this for AB cruisers. This is why is is so important to book very early, as the lower priced modified cabins are often the first to go. Unfortunately few cruise lines restrict these cabins to those with a disability, so many people who are AB ask for them as they know they are bigger cabins.
We had a very good experience on the Zenith in 1995 going through the Canal from LA to San Juan, and also found the crew more helpful than usual. Never had to carry a tray at a buffet for my mother or push my mother's chair for her...always someone who volunteered. They were also more than willing to move our dining table to one closer to a door (easier path of travel), and even opened a door just for us every evening that was not usually used for access to the dining room. The disabled passangers were all invited to sit in the Captain's private lounge (under the bridge) for the Canal transit. The door was too narrow for the chair, but they helped us jerry-rig a way to get in and served us lunch and cold drinks (the captain's private steward). They did need extra coaching in how to get the chair up and down the gangway at ports though and it took them a while to learn. Another plus was that this ship has all the handicapped cabins on one deck, and the hallways on this deck are wider. Also one of the few ships that can accomodate 3 in a adapted cabin. This is rare, but needed if you need to travel with a child or attendant.