How to change default collation for any charset in MySQL?
This is futile.  Any user can explicitly state what collation to use, thereby defeating the 'default'.

A better solution is to upgrade to MySQL 8.0.  It defaults to utf8mb4 and (I think) utf8mb4_0900_ai_ci

mb4 is useful for Emoji and Chinese.

0900 comes from Unicode 9.0, which is newer than 5.20, which is newer than 4.0, which you are suggesting.