If you’re allowed to apply for a 90 days Non O visa directly on your current visa, it’s TM.86. After 90 days you’ll do an extension of stay on a TM.7. You for sure can’t go directly from a ED-visa to an extension of stay.
If you’re on a 60 days visa you can do a 30 days visa extension at your local immigration.
If you’re on a 30 days visa exemption (30 days free stamp) and already done one 30 days extension, you’ll need to leave and re-enter to get a new 30 days visa exemption. (Free stamp).
As said I don’t think you can convert this inside country. If you just cross the border and re-enter on a 30 days visa exemption you apply for a 90 days Non O first on TM.87. After you can do your extensions of stay.
If you already have a bank account you’ll need to bank 800K in your account when you apply and you’ll need at least 15 days left on your stamp. Time should be no issue here as you’re also allowed to do a 30 days extension on your visa exemption.
I guess a regular tourist visa is around € 30-35 (as in Norway). That gives us around 1200 - 1400 thb. An extension is 1900 thb. So it’s not necessarily more expensive but you’ll also need an onward ticket out within 30 days according to formal requirements if you arrive on a 30 days visa exemption.