I'm not sure if that (swapping) is still the case.
I had the good fortune to come across the 787 flight test team for a/c 4, over in Victorville recently and a couple of very nice flight test engineers took some time out of their schedule to show us around their new baby. I asked that very question - are the engines gonna be swappable? - and they responded that it is just a matter of software update and a Rolls engine equipped aircraft, can come out of the workshop the same day with an engine of your choice. As mentioned, the engines are all electrical (no bleed air) and the wiring looms are identical, the engine mounts are the same, so unless Rolls Royce gets on top of this quickly, they may find themselves OUT of the Dreamliner game as operators abandon ship and goes with the competition. I would not like to own Rolls Royce stock if that comes to fruition.