'It' is indeed an impersonal subject in all of your examples, including the last.
Most people use 'for' and 'to' interchangeably in the context you describe, as well. However, there can be a difference.
'An important day to Koreans' means that in the view of Koreans, and according to their opinions, judgments and feelings, it is an important day.
Only slightly differently, 'an important day for Koreans' is more objective. For example, if you were the new president of South Korea, and you believed something was very important in the present or future of Korean people's lives, you could say it was 'important for Koreans' -- whether or not the Korean people were aware of it as yet. To say something is 'important to Koreans' implies that the Korean people feel it is so, while 'important for Koreans' can be used even if the Koreans don't know something is important -- such as a banking system bailout.
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