Long story short, got to get a new passport, now, to travel back to Thailand, all extensions up to date.

Check in at airport should take new passport, but will Thai immigration take new passport, and extension stamps from old passport.

Or is it a trip to Thai embassy to get the stamps changed over.

Just present both passports at immigration on arrival. You will be stamped in in the new one, based upon the re-entry permit in the old one. Then head to your immigration office to get them transferred.

19 minutes ago, kickstart said:

Long story short, got to get a new passport, now, to travel back to Thailand, all extensions up to date.

Check in at airport should take new passport, but will Thai immigration take new passport, and extension stamps from old passport.

Or is it a trip to Thai embassy to get the stamps changed over.

As per the post above, embassy is nothing to do with it.

As you are arriving I think it is unlikely that Immigration will take up time in transferring the stamps - they will probably tell you to go to your local Immigration office.

OP, just enter with both passports.

Assume you have a reentry permit and extension stamp.

Where were those stamps issued.

Specifically was the reentry permit issued at your local immigration office or airport on departure.

Which immigration office do you deal with.