See here.
If memory serves correctly, there was a similar delay before the 2003 California gubernatorial recall, and the Bush Justice Department quickly precleared the election changes to allow the recall election to take place on time. It is not clear if the current DOJ will preclear the date of this special election. Not only is there a partisan valence to this dispute, but there are now allegations that the special election date will disenfranchise some military and other voters.