Ubuntu release update?

Greetings all,

I've not been able to find anything along these lines in the Google
archives or elsewhere. Not sure if it's so simple I'm missing something or
something nobody's ever tried before…

I have an installed base of systems controlled by Foreman that are running
a particular Ubuntu release. I need to upgrade the release on these (going
from saucy to trusty), and I'm not sure the best way to proceed. All of
these hosts were PXE booted & installed via preseed to 13.10 originally
(with Foreman driving the process of course). Install was done from a
local mirror of the Ubuntu archives.

I have Trusty mirroring now, so I'll have that available. I have a couple
of test hosts that I can afford to destroy, re-build to Saucy, rinse repeat
if necessary. I'm not sure if it's as simple as changing the OS entry for
the host in Foreman, then possibly getting Puppet to kick off a apt-get do-release-update on the hosts?

These hosts are users' workstations with local state that needs to be
preserved, so rebuilding all of them (beyond my testing throw-aways) isn't
an option. I need to run Ubuntu's standard upgrade process on them and
deal with (hopefully) minimal fallout from that rather than wiping and

Any guidance would be appreciated.

Best regards,
Zac Bedell