No, that would not quell a thing for me. The biggest issue is not bitrate of SD channels, but the 4:3 aspect ratio for channels which contain widescreen material.
It would not free up lots of hd bandwidth but it would be significant. What would be more significant is that the SD would be much, much better and quell some of the desire for HD on lesser channels.
Example: DIY mostly broadcasts 16:9 widescreen material because they have a HD channel. But because DirecTV only carries the SD version, we get letterboxed 4:3 (black bars are encoded on the top and bottom). Now, when I tune into it on my HD set, I have a choice of stretch-o-vision or watching a show the size of a postage stamp in the middle of set I paid good money for to get the size that it is. I can choose to use my TV's "Zoom" feature, but that just magnifies the fact that resolution was lost on the black bars around the picture.
If they could at least figure out a way to encode anamorphic 16:9 on "SD" channels, I'd be much less annoyed by missing HD channels.