I am going to change employer soon and wonder if I really have to leave the country and apply for a new Non-B visa in the new company's name, or if I can get my current Non-B 'updated' (changed to the new company) at Immigration?

 

Back in the days they didn't care to much about which company you had your Non-B from, but these days they are getting strict about it and even noting on the visa page in the passport which company it 'belongs' to.

 

I read somewhere that if I cancel my current WP I can get 7 days 'extension' at Immigration before my current Non-B will be cancelled. So if I then hand in the new WP application on the same day and hopefully get it done within 7 days, could I then go to Immigration with the new WP and get the Non-B 'updated' to that company and extended?

You apparently have an extension of stay based upon working issued by immigration not a 90 day entry from a non-b visa.

You could change employers for your extension of stay. Your new employer would have to apply for your work permit and get a work permit application approval letter for it. Then you would need a termination letter from your current employer to cancel your current extension and then apply for new extension on the same day it is canceled by submitting the application approval letter and other supporting documents form your new employer. 

After getting the extension you would need to cancel your current work permit and have the new one issued.

The 7 days you can get at immigration are to leave the country after you apply for an extension, pay the 1900 baht fee and be granted 7 day to leave after it is denied. That 7 days cannot be extended.

Many thanks ubonjoe....
This is exactly what I was hoping for. Will give it a try...