I wouldn't of changed our holiday for the world but as a warning..
We did a back to back on Explorer departing from Miami last year, and had expected a nice relaxing day on the change over day. Unfortunately it wasn't quite so simple.
Although we were staying in the same cabin we had to leave the cabin by 9am and weren't allowed back until 1pm. I think this was so they could clean the corridors or something like that.
We were told to wait in one of the lounges until collected - which is basically after everyone else has disembarked. In our case this was around 11:45am.
After getting off we had to complete the full immigration / passport control checks, photos, finger prints etc... which probably took about an hour, before we could get back on board.
Not really the relaxing time we had hoped for I'm afraid. I don't think it would of been as bad if we had at least expected it.
To be fair to RCL a lot of the blame must lie with the Miami immigration services. They weren't ready for the ship when it arrived - and then didn't have enough staff to cope! We had similar problems getting on the ship at the start of the cruise.
Whether you would have similar problems in Puerto Rico I don't know, and even so I certainly wouldn't let it put me off doing a back to back but just be prepared....
PS Only 6 weeks left to Legend of the Seas Med Cruise..time to get the new wardrobe in!