This is part of an in service modification programme that is designed to improve the dispatch reliability of the aircraft. It is very common in new aircraft types as the operators build up experience and report back defects that come up to the manufacturer. The manufacturer then develops a service bulletins (SBs) to fix the reliability issues. It's very similar to a car manufactured modifying a starter motor say after a year of data that the motors are not reliable enough. Usually, the mods are very small in nature, maybe just a software change.
There are over 60 SBs being carried out to the first 4 BA 787s at Victorville (VCV), where Boeing have a maintenance facility. Of those sixty, roughly 50% are hardware and 50% software (just like updating the firmware on you phone/PC). Just in case you think 60 is a lot, they include such items as fitting soft-closers to the toilet doors (before they slammed shut) and disabling the auto-closing of the toilet seat which never really worked as designed.
BJA is complete and BJB is there at the moment. Also at VCV is a Lan Chile 787 having a similar mod package. BJC and BJD are scheduled to go after BJB is complete. BJE onwards are not scheduled to go as they already have a lot of the SBs incorporated from manufacture.
Hope that helps to explain the process.