Good morning. In the event of an emergency the cabin crew would have to move the passengers who were unsuitable to operate an emergency door to an alternative seat. The crew would choose able bodied passengers to replace the unsuitable passengers (ABP’s) who would be briefed how to operate the door and how to remove the cabin crew member from the exit should they become incapacitated in any way that would impede the evacuation of the aircraft. Crew are told in their training to choose passengers who may be in or previously have been in the military, the police force, fire service, passengering cabin crew or indeed anyone who would be confident and physically to assist in an emergency. I do agree with Charles-P that the type of passengers he mentioned should ideally not be allowed to sit by the exit in the first place but I assume the airline “covers” themselves by mentioning on their websites etc that they can move passengers should they need to.