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";