That same change happened to me in 2016 but they moved us from Sunday to Saturday (right after the American Thanksgiving). Fortunately I was able to get our hotel reservation bumped forward a day so it all worked out. Hopefully yours will too.
It looks like they have done away with all but one direct flight to Orlando from Ottawa. Sunday seems to be the only day you can get a direct flight after October 31. Doesn't make any sense. The flight is always full when I've been on it. I guess we just need to let them know by flying with other airlines until they bring it back. (doesn't solve your immediate situation)