Procedure to convert string based fields collations and character sets to any.

change_all_collations.sql

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DELIMITER //
CREATE PROCEDURE changeAllCollations(
	db_name VARCHAR(100),
	char_set VARCHAR(25),
	collation_name VARCHAR(50),
    prevent_key_too_long Boolean
)
BEGIN

	DECLARE done INT DEFAULT FALSE;
	DECLARE tbl_name VARCHAR(50);
	DECLARE changes varchar(1000);
	DECLARE cur_tables CURSOR FOR SELECT table_name FROM information_schema.tables where table_schema=db_name;
	DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

	OPEN cur_tables;
	set changes = "";
	read_loop: LOOP
		FETCH cur_tables into tbl_name;
		  IF done THEN 
			LEAVE read_loop;
		  END IF;
		set changes = CONCAT(changes,tbl_name,"\n");
		BLOCK2: BEGIN
			DECLARE table_done INT DEFAULT FALSE;
			DECLARE col_name,col_type,col_maxlen VARCHAR(50);
			DECLARE cur_columns CURSOR FOR SELECT COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS WHERE `TABLE_SCHEMA`=db_name AND `TABLE_NAME`=tbl_name AND (`DATA_TYPE`='varchar' OR `DATA_TYPE`='char' OR `DATA_TYPE`='tinytext' OR `DATA_TYPE`='text' OR `DATA_TYPE`='mediumtext' OR `DATA_TYPE`='longtext');

			DECLARE CONTINUE HANDLER FOR NOT FOUND SET table_done = TRUE;
			OPEN cur_columns;
			read_table_loop: LOOP
				FETCH cur_columns into col_name,col_type,col_maxlen;
				IF table_done THEN
					CLOSE cur_columns;
					LEAVE read_table_loop;
				END IF;

				# Tolerate the key too long error 
				# This should convert string data types maximum length to 191 which have unique indexes.
				# See https://wildlyinaccurate.com/mysql-specified-key-was-too-long-max-key-length-is-767-bytes/ 
				IF prevent_key_too_long AND col_maxlen > 191 THEN
					SET col_maxlen=191;
				END IF;
				SET @query = CONCAT('ALTER TABLE ', tbl_name , ' Modify ', col_name , ' ' , col_type , '(' , col_maxlen ,') CHARACTER SET ' , char_set , ' COLLATE ' , collation_name , ';');

				PREPARE stmt FROM @query; 
				EXECUTE stmt;
				DEALLOCATE PREPARE stmt;
				set changes = CONCAT(changes,tbl_name,".",col_name,", ");
			END LOOP read_table_loop;
		End BLOCK2;

		SET @query = CONCAT('ALTER TABLE ', tbl_name , ' CONVERT TO CHARACTER SET ' , char_set , ' COLLATE ' , collation_name , ';');
		PREPARE stmt FROM @query; 
		EXECUTE stmt; 		
		DEALLOCATE PREPARE stmt;
		set changes = CONCAT(changes,"\n");

	END LOOP read_loop;

	CLOSE cur_tables;
	SELECT changes;
END//
# ===========> End copying <=============#

# Paste the contents into the sql tab of your database then run (Click Go) to make the procedure.
# To use proecdure just call the procedure like this : 

call changeAllCollations('testdb','utf8mb4' , 'utf8mb4_unicode_ci' , true)

# Pls set the last parameter to true if your intended collation and character set are utf8mb4 based 
#  and your mysql version is older than 5.7.7, otherwise set it false.
# BE CAREFUL! By setting true, unique string based maximum length fields will be trimmed to 191.