Let me put it this way.
The Main Round and all rounds after that work on the same system. Say you got your first preference in the Main Round and you are happy. In this case, you wouldn't change your preferences around thus getting no more offers. Your top pick is offered thus no other degree programs give you offers.
Say you got your second preferences, but you still obviously wanted the first. What happens now is that you keep your preferences the same so in the next round, UAC will see that you haven't been offered a spot so they check if you are eligible in that round. If not, then no offer is given.
To sum up, just rank your preferences in the way you want them to be. If you don't get what you want the first time, keep what you want at the top and then wait for the next round.