Can I change my non-immigrant O visa to a non-immigrant B visa if I get offered a job?
Not sure why you would want to do that because you can legally work on a non 'O' visa as the rules currently stand.
It would be better to get the work permit on the Non-imm O visa then you don't have to rely on the teaching job to keep your legal status going.
You must have a salary of at least 40k baht per month, or show sufficient funds in a bank account in order to be approved and renew.