Well, Google pointed me to someone more clever than me:

http://askubuntu.com/questions/762084/problem-upgrade-15-10-to-16-04

( Now why didn't the do-release-upgrade process tell me there was a /var/log/dist-upgrade/main.log ?)

So, my problem seems to be:

$ grep ERROR /var/log/dist-upgrade/main.log 2016-08-01 11:34:27,800 ERROR Dist-upgrade failed: 'The package 'kubuntu-desktop-trinity' is marked for removal but it is in the removal blacklist.'

hmmm...

Well, I don't have too much time and grief invested in this install so far... So, I'll drop to the command line, apt-get remove kubuntu-desktop-trinity. Then try the sudo do-release-upgrade again.

If that flys, I'll reinstall kubuntu-desktop-trinity.

If ALL THAT works, I'll report back. Jonesy

On Mon, 1 Aug 2016, Jonesy wrote:

I've just installed the Trinity LiveCD

tde-ubuntu-14.04-desktop-r14.0.3-amd64.iso

and performed all the updates. I commented out the entries in /etc/apt/sources.list.d/libreoffice-trinity.list -and- /etc/apt/sources.list.d/trinity.list

and did a sudo do-release-upgrade to upgrade to xenial (16.04) and received the following:

|Checking package manager |Reading package lists... Done |Building dependency tree |Reading state information... Done |Building data structures... Done | |Calculating the changes| | |Calculating the changes | |Could not calculate the upgrade | |An 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 using the |command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. | |Restoring original system state | |Aborting

I don't see how any of the "This can be caused by" pertain.

I have done this process successfully with the _32bit_ Trinity LiveCD

Where to start? Jonesy