(d) One copy of the Initial Apartment Registration form which pertains to the tenant's housing accommodation shall be sent by the owner to the tenant by certified mail. Service of such form pursuant to this subdivision together with the Notice of Initial Legal Regulated Rent shall constitute proper service of such Notice of Initial Legal Regulated Rent under section 2523.1 of this Title. Provided, however, that for registrations served prior to the effective date of this subdivision, any method of service permitted by the DHCR at the time of service shall be deemed to have the same effect as service by certified mailing.