Ryder here, from the Social Care Team at Shopify. Welcome to the Community and thanks for posting! The currency adjustor is automatically locked after an initial sale in order to save you from the consequences of changing your currency. Since you've already refunded your one order, I presume these consequences shouldn't affect your business too much; however, I'd like to clarify them for you now:
Reporting: When switching between currencies, our reports don't factor in the difference between currencies. This means if you had $500 USD processed, for example, it would show as $500 in reports. Should you then choose switch to CAD, the report would still read as $500 for previous processing. Because $1 CAD is not equal to $1 USD, the reports, in theory, will be wrong going forward.
Product Pricing: Similarly to reporting, your product pricing won't automatically be adjusted. Since the dollar amount between currencies is not the same, a product price’s value could increase or decrease if the currency is adjusted and impact their margins. For example, $20 CAD is not equal to $20 USD. This can be managed by adjusting your product pricing accordingly. Note: Refunds processed after a currency change will also be affected by the currency change.
Apps: Some apps may have limitations when it comes to changing your store's selling currency. You can reach out to the App developer to clarify and questions you may have prior to changing your selling currency.
Gift Cards: Gift cards with remaining balances in the old selling currency will not work once the selling currency of the store has been changed. This can be worked around by issuing a free gift card to customers in the new currency.
Shipping: Any shipping rates for a store will maintain the previous currency and will not update automatically. After a currency change, you'll need to delete the previous rates and add them back in manually to list them in their new currency.
I've gone ahead and sent you an email with this information as well. If you're comfortable with the above impacts and adjustments, please respond to me through there and we can look at getting you authenticated for the currency switch.
Hope to hear from you soon!