Sometimes it happens that phpmyadmin (/mysql) don’t allow you to do what it should allow, such as change the encoding of a column (or of a table or of a database), or change the engine of tables.
Then, after many failed attempts via sql query, I found that the easiest solution is
- export the database
- do the changes you want through a text editor, such as Kate, i.g., replacing the old enconding with the new one
- import the (modifyed) database (after deleting/renaming the old one)