My team recently updated TOAD from version 17 to 24. My team installed 32 bit, while I updated to 64 bit (not sure if this information matters).
My team has a shared repository of automation designer apps and since the upgrade I am unable to change the credentials to run the queries in automation designer. Usually I just update the credentials to mine and run the app, but since the upgrade the username is set to my coworker's and it's greyed out so I can't edit it. My coworker has not had any issues.
Rt-click on your app/action and choose "Change connections..." The dialog that prompts for password has been updated to disable non-password fields when it's being used for the password.
If you right-click on the app on the left and choose "change connections", it should trickle down to every action on the right, but maybe it isn't. If you can send a screen shot of your app automation designer, I can check on that.
If that's it, you may have to right-click on the actions on the right and change the connection there too.
Depending on the action, you may have to double-click on it and change the connection. For example, a schema compare might involve multiple connections, so the right-click "Change connections" won't know which one to change. It's really meant for single-connection actions like export DDL, export dataset, execute script, etc.