Outlook is not an upgrade to Outlook Express. The two programs have nothing
to do with one another. Once you install Outlook, you can export your
messages from OE to Outlook (preferred over importing into Outlook to
preserve dates). You can import your Contacts from OE into Outlook.

Its not an upgrade in the excepted sense, since outlook express and outlook
are two different applications.
When you install & configure outlook, express remains, and is still active
to receive mail if you open it.
To port your Express emails to outlook, within Express, Export mail.
To use your old Express address book, within Outlook, Import the address
book

In order to prevent express recieving mail, once Outlook is configured, if
you inadvertantly open it, it would be neccessary to amend its options to
not check for mail, or remove the email accounts.

When running Outlook for the first time it will ask whether you want to
import the data from Outlook Express.

Outlook doesn't upgrade Outlook Express because they are different products.
Accordingly, you will still be able to run Outlook Express and the data will
also remain accessible.