As Seatiger mentioned its up to the airlines to configure the aircraft however there is some static elements of the aircraft that are not flexible. For example the 777 was the first Boeing aircraft that allows for flexibility in the locations of the lav and the galley (not anywhere on the plane but in zones.) Think about how on a 777 3 cabin the galley is right there where you get on at door 2 but on the 2 cabin you go in and turn left to hit the galley - with the DC-10 the galley was always at the same spot, no matter the carier. For example in the 763 the lavs must be attached in one spot so when you think about the mid cabin lavs that structure can get in the way of an airlines seating plans. The water and waste connections are only avalible at one spot so if you want a galley/lav here is where it will go. I assume the new 737s and 787 will have the same type of flexibility as the 777.