Hello Robert C
Hi, I'm Karl and will be happy to help you today.
Can you expand a little on what you mean by "In the Outlook changeover"?
There is no such thing as @hotmail "changing" to an @outlook email address.
Did you create a new @Outlook account expecting messages from the @hotmail account to be delivered to the @outlook account or did you add a new @Outlook alias email address to the @outlook account and then removed the @hotmail email address?
If you created a completely new account, then you will need to log into each account separately. Since only one Outlook.cm based account (@Outlook @Hotmail @Live @MSN) can be logged into one browser session at a time, you will need to log into the second account using a private browser session using the following link
https://outlook.live.com/owa/
If on the other hand, you added an @outlook also address and removed the @hotmail address as an alias, that action is both immediate and permanent. Once removed, the email address can never be recovered by anyone under any circumstances. There are no exceptions to this Microsoft policy. Any messages sent to a removed email address will be bounced-back to the sender with the error that the email address does not exist.
This is described in the following article under the section
<<Remove an email address>>
Add or remove an email alias in Outlook.com
https://support.office.com/article/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2

If you have any questions, don't hesitate to ask, we're here to help you further if needed.