I have an original Non-B visa which prior to last year had been extended for about 8 years based on marriage and money in the bank. I have also had a work permit for the last 5/6 years but have always arranged the visa extension myself based on marriage.

Late last year I let my company sort out the extension using the monthly income from the company for the first time but still based on marriage. Some company paperwork was presented to the Immigration Officer along with evidence of my income and tax receipts together with the usual required documents relating to marriage visa extensions including my wife being present.

The company now wants to transfer me to another company within the group which will involve an amendment to my work permit. However I have been told that I will also have to prepare again to extend my visa and possibly even have to leave the country if necessary and start the whole process all over again as it will be a change of employer. I think the information I have been given is completely wrong as surely my visa extension of 12 months is based on marriage and not employment and therefore has no relevance to the work permit issue. Is this correct?

Correct, your permission to stay remains valid if you have work and a work permit or not.

Yep.

Your permison to stay is based on your marriage.

Is your extension stamped "Thai wife"? If it is the only effect will be when you next extend it as you will have to meet financials and the new company could be an issue so you might have to use the 400k in account two months method.