MariaDB [test]> create table t (a int);

Query OK, 0 rows affected (0.214 sec)

MariaDB [test]> alter table t change column if exists a b int;

Query OK, 0 rows affected (0.044 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t modify column if exists b char(8);

Query OK, 0 rows affected (0.292 sec)              

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t add column if not exists a int;

Query OK, 0 rows affected (0.040 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t drop column if exists a;

Query OK, 0 rows affected (0.089 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t rename column if exists b to c;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'if exists b to c' at line 1