Did you book the cabin directly or was it assigned to you? From what I've read on other boards, the cruise line does not necessarily move passengers from handicap-accessible staterooms; they just tell prospective passengers that no HC cabins are available (unless it is close to the cruise date, as Jim mentioned, when it's opened to general booking). You may want to put an indicator on your reservation that you will move if needed, but that won't guarantee it.
We actually qualify for a HC stateroom, but don't use it for two reasons. First, my son is ambulatory and can get over tub ledges with our help. Second, the one time we did have a HC stateroom (assigned to us), we didn't like it. Among other things, the shower (we had no tub) and bathroom did not have step-up ledges (obviously) and water would run from the shower into the area outside the bathroom door after someone showered.