I recently purchased a Mikrotik RouterBoard to test out the Tenable ByTheWay vulnerability.
https://github.com/tenable/routeros/tree/master/poc/bytheway
It came with routerOS 6.42.1
I was able to downgrade. This was only successful with the command line method described here:
http://forum.mikrotik.com/t/correct-steps-to-downgrade-routeros/118830/1
I successfully went down from 6.42.1 to 6.40.1. I was able to go from this to 6.40.6 (bugfix) Yes, the only way to downgrade versions is to do them version by version!
I cannot go any lower than 6.40.6 (bugfix)
I found this forum entry: http://forum.mikrotik.com/t/downgrade-routeros-from-6-40-5-to-6-38/114638/1 from pe1chl, more specifically this response: You cannot downgrade below the version that comes with a device. When your new device came with a newer version than 6.38,
you cannot downgrade it to 6.38. The minimal version is listed in system->resources.
Can MikroTik staff confirm this?
I’m not looking forward to breaking out the JTAGULATOR and the BUSBLASTER and crowbarring that sweet sweet vulnerable 6.30.1!