From the release notes page#15
http://documentation.extremenetworks.com/release_notes/NetSight/9035078-02_ExtremeManagementCenter_8...
- When upgrading the Management Center server, Application Analytics engine,
or Access Control engine to version 8.0, ensure the DNS server IP
address is correctly configured.
Additionally, upgrading requires an internet connection.
If no internet connection is available, see Upgrading a Hardware Engine.
The problem is that I can't open the linked document.