A simple upgrade can only be performed on a legacy mailbox/email alias combination where the email alias only forwards to the legacy mailbox, and no other addresses. In a simple upgrade, the legacy mailbox is renamed to the chosen alias, and any other email aliases that forward to the legacy mailbox will be changed to point to the new email address.
You will need to update your email client to use the new email address as your login, however the email address' password, and other settings, remain unchanged.
An extended upgrade is performed on a legacy mailbox when an email alias delivers to it, and other addresses, or if a wildcard address forwards to it. During the upgrade process you must already have an email address set up for the legacy mailbox to be upgraded to.
Any email in the legacy mailbox will be copied to the new email address, and aliases that deliver to the legacy mailbox will be updated to deliver to the new email address.
You will then need to set up your email client to use the new email address' details (login and password) for connection.