I cannot downgrade clients from Plus to Essentials. Support even tried and said it is because I am on the new billing system.
QuickBooks Online allows the Accountant, Master and Company Admin to downgrade the account. Let's make sure you're using either one of these login credentials.
When the downgrade button is still unavailable, you can click on the Cancel button and reactivate the account. Then, the downgrade option will be available. Please note that you might need to reenter the billing information.