Can I switch from an educational visa to a non-immigrant B visa in Thailand?

I have personally never seen someone cancel an ED visa extension and apply for a new extension based on employment at the immigration office inside the country.

You cannot get a Non-B visa issued inside the country from an ED visa extension even if you canceled it

What you have to do is get documentation from your school saying you stop studying on xx date, take that to the immigration office about a week before that date, cancel your current extension, they will stamp you in until the termination date on your paperwork.

THEN

you have to leave the country and do one of two things

apply for a 90 day Non-B visa from a thai consulate in a nearby country with documentation from your employer,

OR

exit/re-enter and come back in on a visa exempt entry (free entry stamp) and apply at your immigration office for the 90 day Non-B visa