Theoretically, yes. Practically, no. You can physically uncommit simply by changing the selection in the CYMS tool.
Although I haven't tried it, there is no reason to think you wouldn't be able to do it. Practically, once other schools see you are CTE elsewhere, they will remove you from their WLs, so it is unlikely you would have anywhere else to go, unless a school isn't honoring commitments made elsewhere.
That said, people can and do get cold feet, have last minute emergencies, etc. So, while it is rare, seats do sometimes open up at the last second. Until orientation begins, schools don't know 1,000,000% for sure that everyone is going to show up. That's why they all leave at least a few people on their WLs until that time, and every once in a while you see a report on SDN from someone who received a call a day before or after classes began.