http://docs.openstack.org/trunk/openstack-ops/content/maintenance.html#upgrades has some general information.

Two threads I found interesting on openstack-dev mailing list are:

• openstack-dev] Upgrade Notes from Grenade http://markmail.org/message/haqa5mi3uxo3h3fr

• [openstack-dev] DB issues with Grizzly http://markmail.org/message/5xx3oirxynlgfxvi

If you don't find the information you are looking for I would suggest waiting a couple of weeks until after the OpenStack Summit in Portland next week and then asking on the Operators mailing list for production upgrade tales. I'm sure people would appreciate you sharing what issues you've already encountered in your pre-prod. http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators

I haven't seen an upgrade guide yet, the Grenade project attempts to document the process in code (much like Devstack documents the installation of a system in code). You can check out the upgrade-* scripts at - https://github.com/openstack-dev/grenade

The right answer is: OpenStack with its various Distros doesn't support any warm or even cold upgrades and I couldn't see any plans for it and get any answer from the community to support this in the near future and that's really a pain point which must be addressed in the release after Icehouse.