merger is happening in our customer. Now, we have to update the user details in our app(db) on PROD. But how to test it(proc) before going into prod

Question

2 banks merger is happening for our client. We have to change the data for users in our database. I have created a procedure to change the data in the respective tables. But before going to PROD, how can we test the procedure on the testing environment. Problems here are -

1. we are having a file with the old account numbers and new account numbers for PROD.
2. But in the testing environment, we do not have data.
3. We can not have a staging environment with the prod data to test this.

Do someone have any idea, how to achieve this?