Can I obtain a reentry permit at Suvarnabhumi when transferring from Koh Samui to Amsterdam?

Wasn’t there a newly opened immigration office in Phangan?

Maybe go to Samui a day or so early and go to the immigration office there BEFORE you fly out.

That could work

I don't think you can get one

The re-entry permit counter is in International DEPARTURE #2 after security screening.

Get one from the immigration office in the area where you're at, (if your extension isn't from that office they might require proof you're leaving)

If you're stamped out in Samui it's too late

I do not know how your flight is arranged, or where you will be (legally) departing Thailand. IF you depart Thailand (as in, stamped out) in Samui, you can not get an re-entry permit at Suvarnabhumi. IF you are stamped out at Suvarnabhumi, then you can get a re-entry permit.

I'd save the stress and get a re-entry permit before you leave.

Here's the contact information for Surat Thani. I believe all immigration offices there will sell re-entry permits, and you can use any immigration office.