What lets an app use a secret?

AWS permissions say which identity can do which action on which secret. Give the running app a role that can read only the private setting it needs. Keep test and production identities separate. An app that needs a value does not need permission to change or delete it.

Does changing the stored value change the original account?

A stored version is the copy your app can read. Changing it alone does not change the password or key at the service that accepts it. Check whether managed rotation applies or whether rotation needs a configured function. A replica of a secret can copy a value to another region. It does not create a new database credential.

What can increase the bill?

AWS prices stored secrets and API calls separately. Check the selected region, replica copies and any extra charges for your encryption key or rotation setup. Keep an inventory of the secret references and consuming roles to plan a move without putting private values in a spreadsheet.

How would a test app keep working after a planned replacement?

Sketch an AWS test app that uses a fictional setting from one secret. Write its current and replacement versions as labels only.

Name both changes
Mark where the source service needs a new setting and where the app reads the current one.
Plan the cutover check
Draw the point where the test app would prove it works with the replacement. Keep the real app, roles and values out of the exercise.