They could extend the visa for a year and when they are leaving they could get re-entry visa if they are coming any time soon but if not don't cancel the visa
I once lived in Thailand and it's so difficult to get the Non ED visa inside Thailand it must be from your country of origin and you must have a school you want to go in mind, without that I don't think it's that easy
From experience you cannot change your tourist visa when you get to Thailand especially as a nigerian, so it's better you get a school to admit you and then use it to get ED visa from Nigeria after you have done your NDLEA clearance
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