They would stamp you in on the new passport not the old one. They would make a notation in it that you were entering on a visa/re-entry from the old passport.
Then you would go to the LTR people and get the stamps moved to the new passport
There's nothing that says you can't use that one time 60 day visit thai family extension to "buy time" to apply for another extension.
I see it used routinely when people don't meet the seasoning requirements for retirement/marriage to a thai extensions, they get that 60 days let their funds season and then apply for the year without any issue
It SHOULD work that same way for you, cancel your extension based on employment, apply for the 60 day visit thai family extension, get the documentation from the new employer and then apply for the new extension based on employment.
when that DTV came out they were handing that 5 year visa out to anyone who had an appt for a hangnail from a doc,
Then consulates got harder and started wanting treatment plans, proof of treatment, appt schedule,
SO whether you can or can't do a DTV based on medical reasons would come down to why you are having medical treatment here and how good your documentation is about the treatment.
If you apply and they want more documentation they'll email you, but worst case you apply and they reject the application and you are out the fee
not correct and your status now isn't Non-B, you are on a yearly extension of stay based on employment FROM a Non-B visa you're not here on any visa any more at all
once you get the 60 day visit thai family extension you'd be on an extension of stay for that reason (visiting family), as the extensions were unbroken, it would track back to the extension based on employment and that would track back to an original Non-B visa
you aren't on ANY visa, look at the stamp in your passport, it should clearly say "extension of stay permitted until" and have a date
that isn't a visa
Now, once you get the 60 day visit thai family extension, you COULD get the documentation together, and apply for an extension based on employment (because the only requirement to do that is that what ever extension you're on has to track back to SOME Non-Imm visa that allows employment and at one time you DID have a Non-B visa) even though you've been on extensions many years
The only thing you have to realize is that what ever immigration office you're dealing with to get the new extension based on employment may not allow you to do it once you take that 60 day visit thai family extension (although there is nothing in the rules that prohibits it and many people use that extension to "buy time" to get other
It's the immigration office where you'll be getting the new extension based on employment that will have the final say on whether you can go from an extension based on visiting thai family to one based on employment.