So why exactly are you changing your preferences? Is it because you think you won't achieve the required UAI?
If you really think you'll be going bad, then I do suggest to put a few back-up courses that don't require a significantly high UAI (but which you are still interested in). Don't just change your first preference because you think you won't get it. Many people do that but little do they know that sometimes univeristies provide offers to students in the main round (or later), even though they haven't achieved the required UAI.