Had breakfast with Dave Cripe - NM0S, design engineer on Friday morning at Ozarkcon Conference. 

Dave says "yes" the center frequency can be lowered to 400 Hz but be aware some parameters will also change. Dave is really busy until after the Dayton (Zenia) Ham Convention.  Dave, is also President of the QRP ARCI so he is booked with many challenges right now and cannot do testing to help with the component values for the change of center frequency. 

There was a spreadsheet published but until I get to the QTH and off of the cell phone I cannot lookup any older files.

73,