Hello
Is the old alias an outlook.com/hotmail.com address? Did you end up deleting the old address?
If so, once it is deleted from a Microsoft account as an alias, that account is gone permanently. Microsoft doesn't reuse its own email address, so it cannot be added to created as alias again.
Microsoft explains it here: https://support.microsoft.com/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2 Under the section "Remove an email address", it says: "Are you sure? If you remove an alias that's an email address from a Microsoft domain (like @hotmail.com, @live.com, @outlook.com, or @msn.com), you're permanently deleting the alias and it can't be associated with any Microsoft account again."