Installing the winglets and changing the seat configuration are operations done in two separate steps and by two different contractors.
Every wingletted 757 will have to be taken out of service for the second operation before it goes TATL. I remember reading that NW did this in two steps as it wanted to properly evaluate the effect of the winglets on the aircraft performance w/r to fuel burn etc. Furthermore, at least in some cases, the re-configuring coincides with a heavy maintenance check.
I believe there is a NW press release on this, but I could not find it...