@Jeff ******
He can not convert the extension of stay to anything else.See
"4.3 STV cannot be changed to any other type of visa during the duration of stay in Thailand.
4.4 If the STV holder breaches the entry conditions (for example, if he/she engages in work-related activities), the stay permit will be revoked and he/she will be prohibited from entering Thailand in the future."
(Source:
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/28/special-tourist-visa-stv/)