The short version:

Came to BKK on B visa (on american passport). Decided to study Thai much more before working. Did not get work permit. Enrolled in Thai school full time.

Id like to know if I need to cancel my B visa before applying for an ED visa. Or is there another way to go.

I am aware that I will need to leave the country to apply for an ED visa and have already sorted it out with my school.

Sorry to write so little but I typed this all out once and lost it :/

Feel free to reply with a link if this has already been covered in depth and I missed it. Thanks in advance for your help!

You don't have to change your visa. When the school has the documents required you will just apply for an extension of stay based upon going to school.

All that is required is that you have a non immigrant visa entry to get an extension.