Any period which any person was immediately before the commencement date entitled to count as qualifying service for any purpose (or would have been so entitled if he had been a member), shall, subject to the following provisions of this Part of this Schedule, count as a period to which this Part of this Schedule applies for that purpose (or if he is not such a member, shall so count if he becomes a member in relation to it) and shall so count as a period of the same length as it then counted for that purpose.