I guess that you can paint whatever you like on the side of the hull, whenever you like, just so long as there's only one combination of registration and MSN in the air or with live transponder on the ground at any given period. Possibly just a case of Boeing, with better visibility of the relative completion states, production and delivery schedules for these MSN's than us mere mortals just finishing off the livery enough for each to take it's sequence of test & pre-delivery flights in turn under the same recyclable registration.