Someone in the support thread[1] links to an official Google page[2] where it says under 'Before you unlock additional G Suite features':
> G Suite: You can continue to use your personal Gmail account. However, if you unlock additional features and then cancel business email powered by G Suite, you can't go back to using your original free Gmail address.
> G Suite: You can continue to use your personal Gmail account. However, if you unlock additional features and then cancel business email powered by G Suite, you can't go back to using your original free Gmail address.
Sounds like a terrible 'feature' though.
[1] https://productforums.google.com/forum/#!topic/apps/RIHSJ4LI...
[2] https://support.google.com/work/mail/answer/6243136?hl=en&re...