If:
(a) an instalment period relating to an instalment of a social security payment payable to a person under the 1991 Act commenced before 20 March 2000; and
(b) the instalment period would have ended after 20 March 2000;
this Act has effect in relation to that period and that instalment as if:
(c) this Act had been in force throughout the whole of the instalment period; and
(d) the instalment period had been a period determined under subsection 43(1) of this Act.