There is the option, either at the One-click Actions (called "Refurbish) on Ziphone or in the Advanced Tab to use iErase to erase the bootloader 3.9 baseband and/or "Downgrade 4.6BL to 3.9 and Flash/Unlock 4.03". You will have to identify which option is less destructive, by means of identifying which BL you are actually residing. Proceed with caution, especially the options on the Advanced tab.

These options im talking about are on ZiPhoneOSX 2.6, running on a Mac. I cannot comment on the Windows version, since i have not used one.