Nice answer from https://stackoverflow.com/questions/10859966/how-to-convert-all-tables-in-database-to-one-collation
I adjusted it, so you can specify an OLD collation and new collation.
This way staying away from other collations
-- set your database and new charsets/collations here
SET @MY_SCHEMA = "schema_name";
SET @MY_CHARSET = "utf8mb4";
SET @MY_COLLATION = "utf8mb4_unicode_ci";
SET @OLD_CHARSET = "utf8";
SET @OLD_COLLATION = "utf8_general_ci";
-- tables
SELECT DISTINCT
CONCAT("ALTER TABLE ", TABLE_NAME," CONVERT TO CHARACTER SET ", @MY_CHARSET, " COLLATE ", @MY_COLLATION) as queries
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA=@MY_SCHEMA
AND TABLE_TYPE="BASE TABLE"
AND TABLE_COLLATION=@OLD_COLLATION
UNION
-- table columns
SELECT DISTINCT
CONCAT("ALTER TABLE ", C.TABLE_NAME, " CHANGE ", C.COLUMN_NAME, " ", C.COLUMN_NAME, " ", C.COLUMN_TYPE, " CHARACTER SET ", @MY_CHARSET, " COLLATE ", @MY_COLLATION) as queries
FROM INFORMATION_SCHEMA.COLUMNS as C
LEFT JOIN INFORMATION_SCHEMA.TABLES as T
ON C.TABLE_NAME = T.TABLE_NAME
WHERE C.COLLATION_NAME is not null
AND C.TABLE_SCHEMA=@MY_SCHEMA
AND T.TABLE_TYPE="BASE TABLE"
AND C.CHARACTER_SET_NAME = @OLD_CHARSET
AND C.COLLATION_NAME = @OLD_COLLATION
UNION
-- views
SELECT DISTINCT
CONCAT("CREATE OR REPLACE VIEW ", V.TABLE_NAME, " AS ", V.VIEW_DEFINITION, ";") as queries
FROM INFORMATION_SCHEMA.VIEWS as V
LEFT JOIN INFORMATION_SCHEMA.TABLES as T
ON V.TABLE_NAME = T.TABLE_NAME
WHERE V.TABLE_SCHEMA=@MY_SCHEMA
AND T.TABLE_TYPE="VIEW";