Can anyone advise me on the process to change the visa type from a Non-B category to Non-0 based on marriage and can this be done at Chaengwattana immigration? 

I've answered my own question I think based on a search of this forum. It appears that there is no need to change the visa type, I can extend using Non-B visa based on marriage reason. Is that correct? I've done this in the past but HR in my company seem to think I have the wrong visa category for extending based on Thai wife.

You cannot change non immigrant visa categories without leaving to get a new visa.

You do not have to have a non-o visa entry to apply for an extension of stay based upon marriage.

You can change the reason for an extension of stay from working to marriage. Your current extension would have to be cancelled on the date you apply for the new extension.

You will need to meet the financial requirement to apply for the extension. You would need 400k baht in a Thai bank in your name only for 2 months or proof of a income of at least 40k baht. You can use your income from working to meet the income requirement.