If I understand you right…
Import the updated art into your project. Select the layer/pre-comp/comp (whichever) to want to update in the project timeline. Hold down option key and drag updated file from the project window to OVER the selected layer/pre-comp/comp (whichever) in the timeline and release. If it has same size, layers, anchor points it should update it in place.
Or not. Sorry if I misunderstood the issue.
~CG
Kind regards,
Chris Gomersall
gomersall.chris@gmail.com
817-944-4124
http://www.vimeo.com/channels/texasvideopro