Yes, you go through your TA. You will keep your booking on board discounts, unless the new date you choose is blocked out. I believe the discount will be applied to prevailing rates though. Your TA will be able to walk you through all of your options.
Yes and yes. Your TA will have to handle the rebooking. The 10% discount will apply to the current pricing so long as the replacement cruise is within 24 months of the original onboard booking date and is not subject to a blackout period. You can check current pricing and stateroom availability for the replacement cruise on the DCL website.
THIS! If you do say "cancel" the CM *should* catch that this is an Onboard Booking, and *should* let you know and ask if you want to move it to another date. But what *should* happen and what *does* happen are sometimes two different things.
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