Can a Covid tourist visa be converted into a Non-B visa in Thailand?

The actual answer "can you go from a covid extension to an in country 90 day Non-B visa?" is

It is totally up to your employer if they can provide the documentation so you can apply for an in country 90 day Non-B visa at your immigration office. SOME employers cannot meet the documentation requirements (which is far more stringent than getting a non-B from a thai consulate outside the country)

SO the people you need to really be asking would be your potential employer and the immigration office you're dealing with.

Remember most offices that can do the in country conversion require you to have a minimum of 15 days left on your current stamp to process the visa application or you can't apply

I’m really liking the look of these older style bubonic plague tourist visas myself!

No such thing as a covid tourist visa, presumably you're currently here on an unable to travel due to covid extension?

In answer to your question, it depends very much on whatever immigration office you'll be using and your future employer. It requires more paperwork to do in country and not all employers can provide it, hence a fair few still tell people to apply from overseas.

You cannot change to any visa within Thailand. Visas are only issued by Thai Embassys overseas.