1- 21 days you need to have before your visa expires to convert to non b
2- your company and their lawyer knows documents and process of in country conversion
Keep attested your degree diploma and education qualifications certificate original , from your country department and check your embassy in Bangkok. They will need for work permit
the OP didn’t mention location or company that you can be judgemental of fraud job! It will be wise to get directly b visa which employer can send relevant documents apply from home country and then come. Else it’s not worth