I need to turn OFF multi-currency as I wish to downgrade my account to a standard account. This doesn't seem possible.

Hello and welcome to the Community, accounts-adappta

Once Multicurrency is on, you can't turn it off again, or change your home currency. This is because currency conversion information will have to be accounted for in the system moving forward. 

We've included further information in our guide on how to set up and use Multicurrency in QuickBooks Online. Multicurrency is supported in QuickBooks Online Essentials, Plus, and Advanced, meaning you can upgrade or downgrade between any of these versions. 

If you need any further assistance with your plan, please get back in touch below!

@accounts-adappta 

Once you have disabled multi currency feature, you can't disabled it when you need to downgrade to QBO Simple Start. You can purchase a 3rd party service to convert your data to a new QBO Simple Start.