Comment 48 for bug 186465

doing a sudo do-release-upgrade actually works (doesn't hang) but fails with:
Could not calculate the upgrade

A unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug against the
'update-manager' package and include the files in
/var/log/dist-upgrade/ in the bugreport.

maybe this is what is causing the freeze?
i think that this relates to some bad/old packages with weird dependencies messing things up.
there's another bug relating to this:
https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/181762

hope this was useful....
_J