If you try to book another LL and you are too early, it will show up as you are trying to book it. You will come across a page with your party, and each person will be shown as "Not Eligible" with a text block next to each guess. Within that block it should give you the time you can select your next LL.
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You are correct. When booking LL's you cannot modify return times. If you want a different attraction or return time, you will need to cancel your most recent selection in order to book something else.