Downgrading to other versions below 1.3x is not possible. To make this possible the older firmwares need to be rewritten since 1.3 firmware introduces some new versions of the firmware SDK/kernel (which is locked in the heart of the device).
It is possible if we as user (paid for this device) request for incoming firmware update that Dlink should "return back" option for 3G USB modem funtion? If yes, how to do that? send email to Dlink? or Dlink R&D team never make any survey regarding any issue?