@Sara ******

No need to go to the country of origin.

The Imm. officer said that I can enter Thailand on a "Tourist visa", and then apply to convert it into Non-B, providing supporting documents from the new employer, as well as address registration TM30, education documents etc.

Not sure about how long it takes to convert a visa. I haven't done it yet, cuz still preparing some paperwork.

You should confirm the details with local immigration and labor department in the province where you are applying for a new job.

I have the very same situation, and I was told by the local labor dept. in another province, that they do not accept Non-B issued in Bangkok. In my case the only option is to leave the country.

Better confirm all details with your local Imm office and Labor dept. The process may vary for each province.