I believe that you do need to prepay now if you want a reservation. I assume the change happened because it is so popular and they don't want people to cancel. When you prepay, you do get reserved seating. I believe there's still the option to get there early and hope you can get in, and this could be a way around prepaying, but I certainly wouldn't recommend it.
If you do book a reservation and need to cancel it, be sure to call at least 48 hours ahead of time to make sure your card won't be charged anything.
Hope this helps!