According to my research (watching youtube videos), putting those big turbofan engines (which had to be pushed forward and up to fit) gave the plane different f

According to my research (watching youtube videos), putting those big turbofan engines (which had to be pushed forward and up to fit) gave the plane different flying characteristics which, under the rules of whatever, required new pilots to be retrained to some degree (at least flight simulator training), which made it a real pain for some airlines to ship their pilots off to wherever those simulators are.