This has nothing to do with old vs new hash syntax. You cannot convert output to the new syntax. It will always use the old syntax. What morris expects is probably a JSON string. You can get it by using .to_json.
Use to_json
on your result and pass that to morris.
@users_count = User.group('DATE(created_at)').count.map { |d, c| {date: d, count: c} }.to_json