Will it hurt my search engine rankings if I change the headline of an article 4 days after it's published or even a year after it's published? The content will stay the same, but I want to make some of the headlines on my blog sound more creative.
Yes, it will affect your rankings. Assuming you're using
<h1> tags for your headlines, heading tags tend to carry more weight then most other HTML tags/semantic markup so changing them will influence your rankings. Will it be for the better or for the worse depends on how well the pages ranked before the change and whether the new titles use better keywords then the previous ones.