When you put the footage on the 1280x1080 sequence, the clips on the time line changed some attributes to match the sequence size/aspect ratio.
If you look in the "motion tab" of the clips in the old 1280x1080, you will see that the Size is not 100% and that probably Distortion is not "0" for all his values.
You have to restore these values and the easier way to do it all at once is "Remove attributes".
Otherwise you would have to go clip by clip manually changing those values.
rafael
http://www.nagavideo.com