If you change the sender of the emails in a workflow, while prospects are waiting in an email step, the change will not apply for those prospects, they will get the emails from the old sender.
To make sure that the prospects receive the email from the new sender, you need to remove those prospects from the workflow and re-add them to the needed step.
Click the Waiting button in the needed step. Those are the prospects who are currently waiting in the sending queue.
Click the More actions button.
Select them all, click Manage Prospects > Add to lists, and add them to a new list.
Disenroll them from your workflow.
Open the new list, select all prospects, and enroll in the needed step of your workflow.