I was trying to downgrade from STC v4.20 (or greater), to an earlier release. Now, I can ping the C1 appliance, but I can't telnet, SSH or connect with the STC GUI.
Unlike the SPT-C1 2x10GE appliance, the SPT-C1 4x1GE (STC only) appliance cannot be downgraded below v4.20. Earlier STC software / firmware did not support the 1GE ports, only the 10GE ports.
Once the 10GE port firmware is installed on the SPT-C1 4x1GE appliance, the appliance must be re imaged. This will require a site visit by an Spirent Field Service Engineer or the appliance will have to be RMA'ed.