Yes, they can/do, primarily if the conditions become unsafe for the performers (particularly the stilt walkers). I know that when we were there in early 2020, it was cancelled because of the wind.
I don't know how they handle people who are already booked to ride the floats as the day. But if you have Facebook, you can message the Universal Orlando account and you should be able to get up to date information (you have to go through a bot, but just ask to be connected with a live team member). I know that when we were there, they didn't make the call until pretty close to when people were going to start gathering to ride the floats (so maybe 2ish hours in advance?). At that point, we were in our hotel room, weighing whether we were going to head back to the park for the parade (it was COLD that day). It took about another half an hour or so before the parade was listed as cancelled on the app.