You’ll need to distinguish between an extension (which happens without leaving the country) and exiting and re-entering the country. For the extension, documents MAY be required, and immigration will announce the details soon when the first group needs to extend. For exiting and re-entering, no documents are currently needed.
You may need to provide proof of soft power if you're extending your stay in Thailand; this needs to be confirmed soon by immigration. However, so far, you will not need to provide these documents if you choose to exit and re-enter.