Changing from MAPD to MedSupp and PDP at 65th birthday in May

[BALDELMO]

Client on MAPD now will be 65 in May. If enrolled in MedSupp and PDP effective May 01, do their expenses up to that point satisfying deductible and the $ 4,660 Coverage Gap threshold carry over to the new PDP?

 

[Tscholar]

Client on MAPD now will be 65 in May. If enrolled in MedSupp and PDP effective May 01, do their expenses up to that point satisfying deductible and the $ 4,660 Coverage Gap threshold carry over to the new PDP?

Yes, their expenses will transfer to the new Part D plan:

How to Switch Part D Plans | Eligibility

If you switch Part D plans mid-year, some of your benefits may carry over from your old plan. What you’ve already paid toward your old deductible will still count toward your new one, even if your new plan’s deductible is different. Your progress through the Medicare Part D donut hole will transfer as well.

 

[somarco]

Tiers and copay's will most likely change when he goes on a PDP.

The advice offered by @Tscholar should be helpful for the balance of your questions.