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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def change_column_if_table_exists(self, tablename: str, oldfieldname: str, newfieldname: str, newdef: str) -> Optional[int]: """Renames a column and alters its definition."""

        if not self.table_exists(tablename):
            return None
        if not self.column_exists(tablename, oldfieldname):
            return None
        sql = "ALTER TABLE {t} CHANGE COLUMN {old} {new} {newdef}".format(
            t=tablename,
            old=oldfieldname,
            new=newfieldname,
            newdef=newdef,
        )
        log.info(sql)
        return self.db_exec_literal(sql)