When an A boat which has been originally measured under the previous class rules is remeasured, would that be under the current or the original class rules?

If a boat is being re-measured it is almost certain that this is because its flotation or other dimensions have been changed - that is A.11.1 a), b) or c) apply. A.12. 1 (b) is another possible reason. A.11.2 tells us what can be done when a boat ceases to comply with the class rules (which include its certificate) and option a) is for an official measurer to carry out certification control (i.e. re-measurement) of affected equipment (in this case, the boat).

The boat shall be certificated to the current class rules. However,  Sections D, E, F, G each tell us about possible grandfathering in cases where previously permitted equipment has become non-compliant due to class rule changes.

D.1.1 RULES
The hull shall comply:
(a) with the class rules in force at the time of its initial certification control;
or
(b) with the class rules relating to its certificate; or
(c) with the current class rules.

E.2 RULES
The hull appendages shall comply:
(a) with the class rules in force at the time of its initial certification
control; or
(b) with the class rules relating to its certificate; or
(c) with the current class rules.

F.2.1 RULES
(a) The spars and their fittings shall comply either with the class rules in
force at the time of the initial certification control of the hull or with the
current class rules.
(b) The standing rigging and running rigging shall comply with the
current class rules.

G.2.1 RULES
The sails shall comply either with the class rules in force at the time of their
initial certification control or with the current class rules.