If an individual’s primary insurance amount is determined under paragraph (3) of section 415(a) of this title as in effect in December 1978, or section 415(a)(1)(C)(i) of this title as in effect thereafter, and, as a result of this subsection, he would be entitled to a higher old-age insurance benefit if his primary insurance amount were determined under section 415(a) of this title (whether before, in, or after December 1978) without regard to such paragraph, such individual’s old-age insurance benefit based upon his primary insurance amount determined under such paragraph shall be increased by an amount equal to the difference between such benefit and the benefit to which he would be entitled if his primary insurance amount were determined under such section without regard to such paragraph.