Yes, once a return has been transmitted you cannot make changes. If your return gets accepted, then you would need to amend your return to make a change.
If your return gets rejected, then you can make the change, then re-e-file.
You cannot e-file an amended return, it has to be mailed. You cannot file an amended return until your return has been accepted. You can check your e-file status to see if your return has been accepted yet.
