The mounts alone would not have caused that, see the 747 as an example of an aircraft that can support different pylons based on one of the three different type

The mounts alone would not have caused that, see the 747 as an example of an aircraft that can support different pylons based on one of the three different type of engines used without recertification. What requires recertification is the change in handling specifications (forward mounted engines changes the center of gravity, larger engines which change performance envelope) My understanding is that there is mandatory training for MAX pilots that airlines must track, but it does not require a new type certification. The latter is likely due to regulatory capture, southwest has the largest fleet of 737s and did not want to recertify their pilots[0] [0] https://www.seattletimes.com/business/boeing-aerospace/legal-filing-reveals-southwest-airlines-proposed-a-ploy-to-deceive-faa-on-boeing-737-max/ https://www.seattletimes.com/business/boeing-aerospace/legal...