1080i and 720p on explorer 4642HDC box

I recently replaced my 720P HDTV with a 1080P model.  I know Cox doesn't broadcast channels in 1080P so I have my Explorer 4642HDC box set to select either 1080i or 720P as my output format.   Under this setup 720P HD stations (ie ABC) come in as 720P and 1080i HD stations (ie NBC) come in as 1080i.   This gives an excellent picture for both formats.

Here's my question.  If a non HD station like HBO channel 203 or Flix channel 360 is broadcasting in 480i or 480p, converting the signal to 1080i looks best on my 1080P TV.  However, no matter what  I've tried on my 4642 box those channels always come in as 720P and not 1080i.  It's as if the box makes 720P the default for non HD 480i and 480P channels.  Is there any way to set 1080i as the default for 480i and 480P stations.  I know I can set the box to output every channel in 1080i, but then on sports channels that use 720P like ESPN, the 1080i doesn't look as good as 720P.   Thanks...