Now that they have my money, I would like to have a say if they want to do a modified race. IF they do that, I hope they give us the option to cancel and get a refund.
That would be nice and I think a lot depends on how much they change. If it is minor adjustments like normal course but no characters (though yes I do know characters are a big portion of why people come) or re-routing of the course some (no course maps have been released) they might not. Major changes like moving everything to ESPN they probably would. I'd bet in most cases they would offer options to cancel. They have been good about stuff like that through this whole thing.
Unless disney starts allow character M&G in the parks between now and april they are not going to do them for a race. That is a given and right now it doesn't look like nomal M&G are returning anytime soon. So if you want characters on the race watch what they are doing at the parks.