If you worked for one week at Class A and then signed on for A class credits immediately after you retire this would result in all your modified contributions for the retirement year and the previous year changing to A class credits.
This would improve your contributory pension entitlement.
Signing on for A class credits will not effect your entitlement to S class on the ARF.
In any year you can have S class and A class credits.
It can be advantageous to keep signing on for A class credits as they can help for qualification for 65s benefit. You would only need to work for 13 weeks to then qualify for the 65s benefit.