First of all, it really sounds like a nightmare, I really feel bad for you!   You can read over here my lengthy complaint and story about the upgrade from MS14 to MS15: https://community.meraki.com/t5/Switching/Has-anyone-had-issues-with-Stacks-and-LACP-after-going-from-14/m-p/196404/highlight/true#M13683    There we also had issues with switches being unable to upgrade because the ports were blocked. But I handled that by disabling all redundant connections (which in my case was simply disabling almost all ports on one of core switches, thanks to a rather clean cable setup), then everything regained connectivity and upgraded. Your comment below got me thinking and I will keep that in my mind during next upgrades, since I have the Loop Guard set on all ports pointing towards the core switches (stp root), and maybe that was the reason for my issues. Thanks! @SwitchedAtBirth wrote: We also did some research and found that some folks were complaining about STP Loop Guard can be known to cause issues. We found this enabled on 2 uplinks, disabled and finally, all was well. ... View more