Can I convert a tourist visa to a Non-B visa for teaching in Thailand?

Totally up to the immigration office you're dealing with as far as if they do in country conversions to Non-Immigrant Type visas at that office AND the company you're dealing with as far if they can provide the documentation needed to do the conversion in country versus giving you paperwork to go out of the country and get a 90 day single entry Non-B from a thai consulate.

You need to check with your immigration office AND you need to check with your potential employer. <- Those are the people with the answers you seek.

I did it last year, went smoothly. Get your employer to ring immigration and ask what’s needed. This was in Bangkok, Korat immigration the previous year refused to do it.

And you need certain number of days remaining. Number of days may depend on immigration office.

It is, but you gotta jump over hoops with tons of paprwork by your employer.