I am curious about the "old new planes " that are

I am curious about the "old new planes " that are coming onto the flight lines of these airlines. Do they have to jump through the hoops to get them in operation or are they just added to the currant roster. We are told that our plane is old and we are asked to upgrade in order to help with getting our certificate.
I am just an old drag racer so I have no idea what is really going on. If you or some one could please enlighten me , that would be nice information .

Boss

Boss