Is frame #43 a key frame? When you do this, I don't think that frame #1 will be overwritten, unless the first frame of your selected sequence (frame #43 in this case) is also a keyframe. You can easily tell which frames are keyframes in the animation palette; they're a brighter shade of green. So if #43 isn't a keyframe, make it one before dragging your selection to the left. Hope that helps!