is it acceptable to unpublish your project and publish it again to enter a contest?
No. It also doesn't work if one republishes since the original published date is retained in the databases for the judges to notice. Best is to improve the original ible with new ideas and create a new one.
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What about an "improvement" on an instructable already published?
For example, I did a Vintage radio into an MP3 player. at the time I did not include an amplifier in it, just headphone wire to new speakers inside the radio.. but since then I have thought about editing the instructable to show the changes, or make a new instructable to show the changes made to the radio adding an amplifier. Just not sure..
That's probably OK.