Converting a date to a datetime in MySQL
Create and populate:

mysql&gt; CREATE TABLE grade(d DATE NOT NULL) ENGINE=InnoDB;
Query OK, 0 rows affected (0.02 sec)

mysql&gt; INSERT INTO grade (d) VALUES (NOW()), (CURDATE() - INTERVAL 1 DAY);
Query OK, 2 rows affected, 1 warning (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 1

mysql&gt; SHOW WARNINGS;
+-------+------+----------------------------------------+
| Level | Code | Message                                |
+-------+------+----------------------------------------+
| Note  | 1265 | Data truncated for column 'd' at row 1 |
+-------+------+----------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT * FROM grade;
+------------+
| d          |
+------------+
| 2017-11-22 |
| 2017-11-21 |
+------------+
2 rows in set (0.00 sec)


Upgrade:

mysql&gt; ALTER TABLE grade MODIFY COLUMN d DATETIME NOT NULL;
Query OK, 2 rows affected (0.04 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql&gt; SELECT * FROM grade;
+---------------------+
| d                   |
+---------------------+
| 2017-11-22 00:00:00 |
| 2017-11-21 00:00:00 |
+---------------------+
2 rows in set (0.00 sec)

mysql&gt; INSERT INTO grade (d) VALUES (NOW()), (CURDATE() - INTERVAL 10 DAY);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql&gt; SELECT * FROM grade;
+---------------------+
| d                   |
+---------------------+
| 2017-11-22 00:00:00 |
| 2017-11-21 00:00:00 |
| 2017-11-22 13:56:37 |
| 2017-11-12 00:00:00 |
+---------------------+
4 rows in set (0.00 sec)


Downgrade:

mysql&gt; ALTER TABLE grade MODIFY COLUMN d DATE NOT NULL;
Query OK, 4 rows affected, 1 warning (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 1

mysql&gt; SHOW WARNINGS;
+-------+------+----------------------------------------+
| Level | Code | Message                                |
+-------+------+----------------------------------------+
| Note  | 1265 | Data truncated for column 'd' at row 3 |
+-------+------+----------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT * FROM grade;
+------------+
| d          |
+------------+
| 2017-11-22 |
| 2017-11-21 |
| 2017-11-22 |
| 2017-11-12 |
+------------+
4 rows in set (0.00 sec)

mysql&gt; 


I deliberately included trying to put a DATETIME into a DATE column to show what happens, and to show the Warning that occurs.