What form do I need to close my current ED visa and apply for a new one in Thailand?

there is no form to cancel the current extension of stay you're on, just the documentation from the school you're leaving stating when your last day is.

For the new extension you'd fill out a TM7 application for extension form.

You'll cancel the extension from the school you're stopping studying on, and you'll immediately apply for a new extension with the new documents from the school you're starting studying on.

Hopefully both schools are in the same area and you use the same immigration office

Whether you can do it without exiting/re-entering the country and starting over depends on the documents from the schools you're leaving and starting at and your immigration office.

They might not let you switch schools on the current extension you have. You might need to cancel it, then bounce out/back to get a free entry stamp and THEN apply for a new in country 90 day Non-ED visa