Thanks for sharing your query with us here in the Shopify Community.
It sounds from what you've shared here that you'd like to add a new bank account to your Shopify Payments account, but are being required to enter the existing info as a security step.
Would you be able to check for this info from the bank you're using at the moment?
There are a few ways you could access your account info from your bank:
A Void Cheque: a blank cheque from their bank contains the account numbers
Online Banking: account details may typically be viewed online
Direct Deposit form or Bank statement: forms are usually available for download
Contact Bank: call/chat with the bank to clarify current or old account details
Let me know if you're not able to access this info in any way for whatever reason, and we can see what else we might be able to do to help out here!
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