With the release of Team Foundation Server Update 3, security was enhanced by enabling basic authentication between TFS and the Search service. Before there was no security enabled out-of-the-box.
This means that when you try to upgrade to Update 3, you are required to provide a new user/password combination:
More information: https://blogs.msdn.microsoft.com/devops/2018/09/13/search-msrc-fix-for-tfs-2017-update-3/