Sending, receiving, and organizing email in Outlook.com
Is your existing address a Microsoft-owned domain (outlook.com, hotmail, msn, live) address? If not, Microsoft is starting to ask users to add an outlook.com address to the account. Non-Microsoft addresses are only used to sign in and are not used for email. If you sign into outlook.com, the address will show as outlook_[ big ugly code ] @outlook
Go to https://account.live.com/names/manage - are the old and new addresses listed? If no, sign out then try signing in with the existing address or open a private browser window and try signing in with the existing account address.