Basically, re-shopping is making a new booking so the on-board booking will be based on cabin availability and pricing at that time. You'll book at the current prices and can only choose from available cabins. So, if your cabin category isn't available, you'd select from those that are.
What I'm not sure about is whether you could go back to your original cabin category and specific room when the 2 reservations are reconciled. That is, when your original reservation is cancelled, it would likely, but not guaranteed, free up that cabin so that you could then switch back to it. But pricing would be the pricing that was in place while you did your OBB.
Regarding the reconciliation of the 2 reservations - I've experienced one thing and read somewhat differently here. I booked through a TA, so I had to wait until my cruise was over and then my TA cancelled my original reservation, and then switched the new one to my original cabin. But note that my original cabin category WAS available for my OBB and the price hadn't gone up. So, my OBB was for the same category but just a different specific cabin.
I've read that sometimes the OBB agent can do the reconciliation if you didn't go through a TA, and even sometimes when you did. My experience was with booking through a TA that it had to be reconciled after the cruise.
Something else I encountered was that Main Dining was no longer available when I re-shopped onboard and made a new reservation. So my new reservation had 2nd dining and my original one had main dining. My original one was cancelled but I couldn't switch to main dining because that goes by a wait-list and I thought that since I had it originally, I could still keep it. Just something else to be aware of it the cruise has filled up considerably since your original booking. Again, this goes along with you can only book from options that are available at the time of your onboard booking. For me, Main Dining was no longer available so I couldn't get it.
