while holding shift in the time slider, drag-select all the keyframes you wish to shift. this area of the time slider will turn red, and at either end there will be a black arrow... after releasing shift, dragging using these arrows will stretch your keyframes out, but be careful cause they won't always fall on full frames (instead of being on frame 15, a keyframe might be on frame 15.64)... you might have to go into the graph editor and just fix those.