Hey there ARTAartsdelatable,
Thanks for reaching out to us here. QuickBooks Online is a dynamic program that helps keep your books balanced. I would be happy to provide some information.
Once Multicurrency has been turned on, it can't be turned off. This is why you're unable to downgrade. You'll need to upgrade or downgrade to a subscription that supports multi-currency. This is because QuickBooks changes the way it calculates numerical values for your company file when multi-currency is enabled. In order to successfully change plans, you would need to create a new account and import your data. However, you will no longer have access to the multicurrency feature and this will affect your purchases in JPY as well. Before making a decision to downgrade your current QuickBooks Online plan, I recommend looking into third party apps for inventory management, to ensure it's the best fit for your business needs. Once you've decided, here's how to cancel your current QuickBooks Online account and export your data.
If you have any other questions, feel free to reach out to us here.