My source is [1], specifically at about 5:05. Alaskan airlines chose a 178 passenger configuration for their 737-9, and so are not required to have a mid-cabin

My source is [1], specifically at about 5:05. Alaskan airlines chose a 178 passenger configuration for their 737-9, and so are not required to have a mid-cabin exit door. Lion Air's chosen to go with a 221 passenger configuration, and so are required to have an operating door. Obviously changing up the number of seats isn't done on-demand, you'd need to go for a refit/maintenance cycles. But if Alaskan decided to change density, or sold the aircraft to someone else who decided to change density - then they could go and do this. [1] https://youtu.be/nw4eQGAmXQ0?t=305 https://youtu.be/nw4eQGAmXQ0?t=305 "The Boeing 737 Technical Channel"