Ash here from the Shopify team. Welcome to the Shopify Community!
I understand you're trying to change the selling currency on your store, but have already made a sale. Before we begin - congratulations on the sale! Once a shop makes its first sale, the currency selector in the Settings > General section of the admin becomes locked for a number of reasons. Our team can remove this lock, but it's important to understand why it's applied in the first place to avoid future issues. The main impacts on your store when changing selling currency are:
Our team will need to access the back-end of your account to unlock the currency selector, so I've sent you an email to the email address associated with your Shopify Community account for authentication. Once you've decided how you'd like to proceed, please reply back to that email and we can take the next steps together.