Not correct. Switching to 8PSK may or may not improve quality - it just gives an opportunity to Bell to increase the bitrate and improve the quality, but they may choose to cram more channels per transponder instead without change to the bitrate.
The exact same thing applies to using MPEG4 - it may or may not improve quality.
My guess is that if they switch quickly to 8PSK at least at the beginning the bitrate and therefore the quality of the HD channels will improve across the board.
Oh, and I guess you meant 1080i, Bell has never sent 1080p video.