(16)When developing new TSIs the aim should always be to ensure compatibility with the existing authorised system. This will help to promote the competitiveness of rail transport and prevent unnecessary additional costs through the requirement of upgrading or renewal of existing authorised subsystems to ensure backward compatibility. In those exceptional cases where it will not be possible to ensure compatibility, TSIs may establish the framework necessary to decide whether the existing subsystem may need to be re-authorised, and the corresponding deadlines.