Changes happen whether you like it or not. There are different kinds of changes, so pick a subject of change and create a cover based on it. For example: 'Changes of the Season'; 'Changes of Love' and so on.
Examples:
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Note, these are examples you can use, but you can also do your own research.
✎ Tips from Judges
x_Apple_Bottom_ x: Give the characters a story don't just make the cover, because you like the character and they are cool make the cover as if it's a summery of your story.
re-ika: For this theme, it's important to show a story. But that doesn't mean you need to make anything too complicated. The 'change' something big or small. You're being judged on creativity not complexity.