17.—(1) In the case of an existing process, the prescribed date is—
(i)in a case falling within paragraph (2), the date at which the change mentioned in that paragraph is made unless later than the date applicable in accordance with sub-paragraph (ii);
(ii)where sub-paragraph (i) does not apply and subject to paragraph 19, the day after that on which the period for applying for authorisation in accordance with the Table in paragraph 18 expires.
(2) A case falls within this paragraph if the person carrying on the process makes a substantial change in the process on or after 1 April 1992 and that change —
(i)has not occasioned construction work which is in progress on that date; or
(ii)is not the subject of a contract for construction work entered into before that date.