In general, you could downgrade an ERS, but would loose the configuration. But that would not be an issue for an RMA switch. As mentioned sw 6.x is VSP code and not BOSS. You may have received a VSP4800 instead of an ERS4800. Latest is 5.12.x.
If it's a single unit, we need the MAC address to reset the password. If it's a stack, we need the MAC address of the base unit. We have an internal tool to generate a backdoor password.