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If a Mod+ changes the content rating of your item, only a Mod+ can change the rating from then on. So if you want to request the rating be changed, you can post the request in "Content Rating Support" .
When you edit your contest forum page, you need to make sure the content rating is still set at 13+ (and do not lower it to E) in order for the changes to apply. If you change the forum rating to E, you will get the error message you received.
Hope that helps.