Extension of validity certificates will be issued if you have not changed your original mode, ie your original validity date plus five years. If you change your mode, the certificate issued will be new, ie five years validity from the date resulted.
Candidates who are applying to extend the validity of an existing certificate should note that if an extension is required to all subjects currently held, each subject would require to be re-taken regardless of the date achieved. For example, if you achieve Core, Road and All Classes in September 2010, each subject pass is valid until November 2015. If you then achieve Rail in July 2012 the validity of this pass will also be November 2015 (not July 2017). If you chose to revalidate Core, Road and All Classes, the Rail pass will not be carried forward.