Can a composition be scaled proportionally shorter?

• Create a new comp that is the same length as your original composition
• Add the original comp to the new one
• Press Alt/Option + Ctrl/Cmnd + t or go to the Layer menu and enable Time Remapping
• Press K to go to the last Time Remapping keyframe
• Press Ctrl/Cmnd + left arrow to move back one frame
• Click the diamond in the center of the keyframe navigator group in the timeline (< ◊ >) to add a new keyframe
• Press K again to go to the last keyframe in for time remapping and click the ◊ again to delete the last keyframe
• Drag the new last keyframe for time remapping  to 9:29 (assuming 30 fps) in your timeline
• Press N to set the Work area so the time display shows 9:29 and the end of the Work Area is at the first frame of 10 seconds
• Trim the comp to the work area

Because you eliminated the last keyframe there will still be an image at frame 10.00, but if you trim the comp there, the comp will be one frame too long. If you chose not to set a new keyframe on the last visible frame of the comp and delete the keyframe that Time Remapping sets there will be no image at 10:00. I hope that makes sense. Most folks don't really understand why time Remapping sets the last keyframe after the video is over.

Your entire animation will be retimed. If there is no video footage in the original comp there will be no blended or interpreted frames, everything will be perfectly retimed. If you have a video in the original comp you'll either have to live with the speed changes and audio pitch changes or figure out how to change them. If you have audio you will have to silence it in the nested comp and add it to the new comp and edit it for time.

Keyframes are always set relative to time, not to frames so if you had a 1 fps comp that was 10 seconds long and you animated the position of a layer from one side to the other over 10 seconds. when you played back the comp the layer would jump from one position to another every second. 

If you nest that comp in a comp that is 60 fps and previewed the motion the layer would move from 1 side of the composition to the other 60 times a second, the motion would be perfect and there would be nothing to do.

The only time this does not work is when you open up the Nested comp (pre-comp) and go to the Advanced settings and select Maintain Comp Frame Rate when nested or in the render cue. There are almost no occasions when this is a good idea.