http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/0e970eccc4b0719e862582cf005b04bd/$FILE/A28NM_Rev_38.pdf
This link is what governs safety. (Airbus A320 family)
If the FAA wants to revise seating arrangements then they need to revise the TDS which they have already approved.