Do I need to cancel my ED visa before applying for a Non-O visa in Thailand?

When you got your education extension, you signed a piece of paper that said you would notify immigration of any change in status. Ending your studies is a change in status.

You need to get a letter from your school that states the last day of study (I would make it a couple days after you plan to leave Thailand if they will do that) so you can have some leeway. Then take this letter to immigration about a week before the date on the letter, and they will cancel your extension and then stamp you back in until the date on the letter.

If you fail to do this, and you go to immigration in the future, for example for your extension based on your non-O visa, they can charge you 20,000 baht for failing to cancel your previous education extension. They can also prevent you from applying for your extension until you officially cancel your education extension, even so far after the fact.