Immigration, issue visa extensions of 7 or 30 days.
However, if you want to change your visa Type. (To a long stay visa)
Exit Thailand, apply for DTV Medical Visa, if you already have an existing health issue, you can get a letter from a Doctor in Thailand which supports the DTV.
5 year visa, 10,000 THB visa fee.
You must be physically outside Thailand to apply for DTV.
Apply at the official website:
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Processing can take up to 3 weeks, so plan to stay 4 weeks in a neighboring country.
(Ensure you choose a Thai consulate located in the USA such as Washington, this way your e-visa will say "place of issue" "USA" state, you'll avoid the scrutiny of visas issues in SE-Asia)
If you don't want to leave Thailand,
Business Visas can be arranged through an agent & the visa can be swapped over without needing to exit Thailand. (Far more complex & unnecessarily costly)