What Precautions Should I Take When Changing from an ED Visa to a Volunteer Visa in Thailand?

As a rule they allow reason for extension changes when you have 30 days or less left on your current extension IF you meet the requirements for the new reason you're applying for an extension.

You would get a letter from your school stating you are stopping studying on xxx date, and you would get ALL the documentation required from the place you volunteer at to get an extension.

You cancel the ED extension and immediately apply for a new extension based on volunteering.