You have a Designer workflow with Impersonation step in it. You publish it to test site with your(developer’s) account, test it and it works great! Now you need to publish it to production site with an other account or a service account (like i do). Now, you want to login as a different user into SharePoint Designer!
You cleared your cache, flushed everything, cleared Designer cache and it does not allow you to login with other account. Here is the trick – look for the tiny little person symbol towards bottom left of designer, when you scroll over it, it says you are logged in as Domain\user. click on that symbol and voila! You get to login as a different user. Hope it saves some time for you.