The amendment or repeal of a form prescribed under section 8, or a provision of Schedule 3 that prescribes an expression or specifies a kind of gift or benefit for the purposes of section 11 (2), 12 (2) or 13 (2), does not:
(a) confer any additional authority on an attorney under a power of attorney that was a prescribed power of attorney in force immediately before the day on which the amendment or repeal takes effect (an
"existing authority" ), or
(b) remove any authority conferred on a principal by an existing authority, or
(c) otherwise affect the continued operation of an existing authority.