depends what visa you have. Tourist visa would require a return or onward ticket. Some form of residency (eg retirement) would be OK with 1-way ticket.
An exemption isn't a visa, so you can't "change" it. You may be able to apply for an actual visa depending on the type you want and on your circumstances.
I've worked it out into British pounds that a 15 night stay in a hotel charging 40,000 baht is about £60 a night at current rates. Taking into account meals, covid tests and transport, security and so forth ... that's not bad.