Choosing a seat or purchasing a paid seat is not a check-in process.
During the online check-in process, you can choose an available seat by viewing the seating plan of the flight.
While many airlines do not charge a fee for online check-in, some airlines may charge for specially preferred seats.
Due to technical problems, the airline company from which you have completed your online check-in process and purchased your seat has the authority to change your seat number.
If there is any change in your reservation after you’ve checked in and created your boarding pass, the boarding pass you’ve created will not be valid and you will have to make a new check-in and create a new boarding pass.