How can I switch my work visa from Company A to Company B in Thailand without leaving the country?

Sorry but that's wrong Bob. I work for a US company, at any given time they have over 500 people on assignment in countries around the world. They have an entire team that looks after us and they use in country agents to arrange Visas. Coming to Thailand I gave my employer a copy of my passport and last 3 months bank statements. I did nothing else, never logged in to the immigration portal, signed no papers, never even paid a fee. My employer dealt with their local agent. I had one email from the agent advising me to stay put in AU until the visa was approved and about 4 weeks later the agent emailed me the approved visa. They are located in Thailand , still provided the same required paperwork, they even applied through the Sydney embassy and there is 100% nothing at all illegitimate about it. Not everyone who uses an "AGENT" is trying to game the system.

@Bob *********

The opposite in fact. Agents who vet applications and offer full refund of their fee plus Embassy application fee are actually encouraging applications.

@Robert ******

Correct you must not be in Thailand when the application is submitted and definitely should not arrive before it is approved however there are legitimate agencies that can prepare and submit the application on your behalf without breaking any rules.