Hyde here from Shopify.
You're absolutely right that if you have already made sales, even a test order, we disable the currency change function automatically. The reason for this is so you can reach out to our support team and chat with us about the effects this will have on your business if you were to change currencies after sales have been made. I'm going to list them below:
With Shopify actively encouraging merchants in some instances to adjust currency for new features, we want to make sure merchants are properly aware of any potential areas they need to be aware of for their account.
That being said, there are also some possible workarounds!
I've sent you an email in relation to this, please let me know if you still want to go ahead with the currency switch in response to that email.
All the best, Hyde.