Unclear.
I believe the 3rd class remains in effect and any requirements to maintain that 3rd class must be maintained. There is no mechanism to surrender 3rd class and move straight to Basicmed that I'm aware of if. If you surrender it, it's treated as withdrawn and you'll lose Basicmed.
But I also cannot think of a situation where this would be needed. Why would you give up a medical while it was still valid?
I guess I was confusing things. I'm not talking about surrendering or giving up the medical. I was trying to ask about letting it expire without renewing it. But if the medical requires and SI to stay active, would NOT renewing the SI and letting the SI expire be the same as letting the Class III expire?
It's a timing issue:
Suppose Class III is a 2 year that renews (physical required) on even years:
2020, 2022, 2024, ...
OSA SI is an annual renewal (on even years the SI and the physical are both required):
2020, 2021, 2022, 2023, 2024, ...
Note that the Medical paperwork will always say "Not valid for any class after xxx/202x" where 202x is the SI renewal date, annually, even though the physical is only required on a normal 2 yr schedule.
If you want to go Basic Med in 2021, the Class III is still good (no physical required) as long as you get the SI renewed.
So - get Basic Med in 2021 before the SI expires and skip all the paperwork and expense of the SI renewal in 2021. Now what? The Class III requires the SI be renewed, but you skipped it. Will the Class III expire a year early? I get that you can have both a Basic Med and a Class III while you wait for the Class III to expire, but what happens if you need an annual SI but skip it while the Class III is still in effect? I know it isn't valid, but it also isn't really expired, so will that cause an issue that would invalidate the Basic Med like a denial would?
Or do you have to wait for the even numbered year where both the SI and the physical are both due and then let everything expire at the same time?