The AltBoot procedure works just in AltaLink and other Xerox ConnectKey models, but for the VersaLink that is not functional, it is a procedure who involved the Diagnostic menu (Download Mode in Special Boot Menu) accessible just for service engineer. A functional machine can be upgraded through EWS.
Try a different USB Key with the firmware version that you know was working.
You still should be able to boot off the USB key. Once the Firmware is loaded, then you can attempt to load the newer FW.
You need a tech to restore the machine software, because he had rights to enter in Special Boot Mode for reinstalling the software.