You can re-book as two different packages. We did this in 2006. We stayed at POP (not an option with the land sea package) and then went on the cruise. POP transfered our luggage to PORS and we transfered ourselves via the bus with a stop at DTD. If you stay at a resort that does pick ups for the cruise, this is not a worry.
Look at the costs of both the land sea package and a seperate package for each and compare your bottom dollar! In our case, it was a substantial savings to book seperate.