Well, this is news to me, but DevNet currently has the ACI simulator available for download! Its in 4 parts, and is about 20GB in size, but this gives everyone an AMAZING opportunity to get their hands on ACI, and play with it a bit. You can download it here. Ill post some updates once I get it installed, and can play with it!
So last week I spent quite a bit of time trying to upgrade ACI 1.1 to the latest version of 1.3 with no success. Luckily, I was in the initial stages of building out the initial fabric, because the failure that I incurred forced me to 'setup-clean-config.sh <filename>' on the leaf and spines, and 'erase-config setup' on the APICs. This forced me to go back through the bootstrapping process on the APICs, and a rediscovery of the fabric. I did this 3x, until I finally punted, and tried to upgrade to 1.2 (which was successful).
I wish now, that I would have recorded the error I was getting, but what would happen is the first APIC would try to upgrade, and simply fail. It was hung, and none of the others would start the upgrade as the 'token' was not being passed to them. I tried to reload the failed APIC, but with no success. After the reboot the 'controller level' was set to 1.3, and that APIC was still at 1.1, so it would not join the cluster, and the only message I got from it was 'the messaging layer was unable to deliver the stimulus (rejected, peer is not ready).' This brought me to the Cisco community article: https://supportforums.cisco.com/discussion/12984291/messaging-layer-was-unable-deliver-stimulus-rejected-peer-not-ready and my ultimate conclusion..that I was stuck!