According to The Weather Channel's website, Universal's area code has only a 10% chance of rain on the 29th. Not sure where you're seeing 40%...
None of us can predict the weather here, so the best thing is to follow the weather on that day and see what the forecast will be like for each hour. If it gets bad enough, some of the outdoor rides with close down due to lightning in the area or wind. I doubt that will happen, though. If it rains, just bring or buy a poncho.