How can I edit already existing credentials? Our company was acquired by another company and my Exchange usernames has changed.

Since our company was acquired by another company, my Exchange usernames has changed. I went to System Preferences > Internet Accounts and selected the existing Exchange account, but when I click on Details, all I get is the ability to change the Description, Name and Password, which does me no good. I can't change the username or anything else related to the account, like servers.