Yes you can modify your initial 3 LLMP at any time before you tap into your first selection. However you must respect the Tiers until after you tap in to your first selection.Sorry if this has been answered. If you have already prebooked your 3 LLMP's for a future day, and paid... can you go back later (prior to your stay) and try to modify those 3 again before you get to WDW?
If you purchased a lower priced LLMP park and then want to change to a higher priced park there have been reports that you are charge the incremental price.
Dave