I’ve tried to replace my old bank account with a different one and when I go to enter my old routing and account details it says that it doesn’t match what Shopify have on file.
Thank you for the screenshot showing the error message you are seeing in your admin. In this situation you will want to reach out to for support through our authenticated support channels (email, live chat, phone).
Once connected with authenticated support they can escalate your request to our billing and payments team for assistance with this.
To contact our live support you can either follow this link: Shopify Help Center - Contact Support or you can reply to your most recent billing email requesting support. Please rest assured that any failed payouts will be held in a pending state until the new bank account details are in place.
If you feel that the bank details on file were placed their fraudulently or are concerned about 3rd party access to your account, please reach out to our live support through the Help Center and let us know.