From what I have read, upgrading from the Open Source Edition to the Network Edition is straightforward. Just downloading the network edition, extracting it, unpacking it, and running install.sh. I have done those steps.

Running install.sh outputs:
Operations logged to /tmp/install.log.10270
Checking for existing installation...
    zimbra-ldap...FOUND zimbra-ldap-5.0.14_GA_2850.UBUNTU8
    zimbra-logger...FOUND zimbra-logger-5.0.14_GA_2850.UBUNTU8
    zimbra-mta...FOUND zimbra-mta-5.0.14_GA_2850.UBUNTU8
    zimbra-snmp...FOUND zimbra-snmp-5.0.14_GA_2850.UBUNTU8
    zimbra-store...FOUND zimbra-store-5.0.14_GA_2850.UBUNTU8
    zimbra-apache...FOUND zimbra-apache-5.0.14_GA_2850.UBUNTU8
    zimbra-spell...FOUND zimbra-spell-5.0.14_GA_2850.UBUNTU8
    zimbra-proxy...NOT FOUND
    zimbra-archiving...NOT FOUND
    zimbra-convertd...NOT FOUND
    zimbra-cluster...NOT FOUND
    zimbra-core...FOUND zimbra-core-5.0.14_GA_2850.UBUNTU8
ZCS upgrade from 5.0.14 to 5.0.15 will be performed.
Checking for available license file...

ERROR: The ZCS Network upgrade requires a license to be located in
/opt/zimbra/conf/ZCSLicense.xml or a license previously installed.

The upgrade will not continue without a license.

Your system has not been modified

New customers wanting to purchase or obtain a trial license
should contact Zimbra sales.  Contact information for Zimbra is
located at http://www.zimbra.com/about/contact_us.html
Existing customers can obtain an updated license file via the
Zimbra Support page located at http://www.zimbra.com/support.
I have requested the trial license, but from the documentation I should be able to just install it. I am running on ubuntu 8.04 lts based linux.

Any help would greatly be appreciated!