If your only income is from your retirement, you don't have any "earned" income, so you don't qualify for the Foreign Earned Income Exclusion.
So unless you are also paying income tax to Thailand on your retirement income (in which case you could use the Foreign Tax Credit), your tax return will be the same as always, with nothing special done.
When you say "expat", so you mean you are a currently a US Citizen who lives outside of the United States? Or do you mean you formally renounced your US Citizenship and are living outside of the United States?
I'm a US Citizen and own two houses in the USA and go back to visit family once a year for about a month.
I always file my income tax as if I was living in the USA but was told at one point I could file as a Expat and receive a tax break. But I do not work in Thailand and only rely on my retirement income from the USA.
You should be aware of the tax treaty between the US and Thailand: <a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-trty/thailand.pdf">https://www.irs.gov/pub/irs-trty/thailand.pdf</a>
If your only income is from your retirement, you don't have any "earned" income, so you don't qualify for the Foreign Earned Income Exclusion.
So unless you are also paying income tax to Thailand on your retirement income (in which case you could use the Foreign Tax Credit), your tax return will be the same as always, with nothing special done.