Ok, so the shared port(s) are not connected to the EAPS master(s)? If you upgrade within a major EXOS release, I believe you should be fine (within EXOS 30.x for instance), but bigger hops may be troublesome. I would get some advice from the TAC in that case. As a baseline, upgrade everything to the latest in the master release (latest 22.x if you're on EXOS 22) and then upgrade to the latest in the next step (30.x in that case) and so on until you reach the desired release. Landing on the latest in each subsequent major release will give you maximum feature parity with the next major release.
If you want to play it safe, disable ports so no loops can occur when a switch is rebooted. You will lack redundancy anyway during the reboot. Do the disable port in an adjacent switch, not in the switch being rebooted. Also disable a strategic port in the second ring as well while rebooting the switches in the first one and vice versa. That will prevent any strange mishaps.
On a general note, I often like to reboot the units before an upgrade if the units have been up for quite some time. That gives you the most stable state in terms of memory leaks and other bugs.
