Ash here from the Shopify team. Welcome to the Shopify Community!
I understand that you've made some sales, but wish to change your selling currency at this time. I'm happy to help out! Once a shop makes its first sale, we lock the currency selector for a number of reasons. I can remove this lock, but it's important you understand why it's applied in the first place. The main impacts on your store when changing selling currency are:
Since actioning this request would require me to access your account, I've sent you an email to the email address associated with your Shopify Community account for authentication. After familiarizing yourself with these impacts, let me know if you'd like to proceed by replying to that email and we can take the next steps together!