I just did this not too long ago. I asked to keep the same booking number. The $100 p/p penalty was applied towards my deposit of $500 bringing the total deposit paid on the new cruise to $700
I changed to a complete different itinerary on a cruise. I called and asked about keeping the same booking number and RCL charged me an additional $200.00 on top of a previously paid deposit and applied it to the cost of the cruise