Assuming he's asking to convert his "temp1" / "temp2" fields from char to float types, yes, it is possible. In either direction too.
mysql> describe temperatures;
+-------+-----------+------+-----+-------------------+-----------------------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-----------+------+-----+-------------------+-----------------------------+
| date | timestamp | NO | | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
| temp1 | char(10) | YES | | NULL | |
| temp2 | float | YES | | NULL | |
+-------+-----------+------+-----+-------------------+-----------------------------+
3 rows in set (0.00 sec)
mysql> select * from temperatures;
+---------------------+-------+-------+
| date | temp1 | temp2 |
+---------------------+-------+-------+
| 2013-01-16 10:51:58 | 1.1 | 2.2 |
+---------------------+-------+-------+
1 row in set (0.00 sec)
mysql> alter table temperatures modify temp1 float;
Query OK, 1 row affected (0.02 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from temperatures;
+---------------------+-------+-------+
| date | temp1 | temp2 |
+---------------------+-------+-------+
| 2013-01-16 10:51:58 | 1.1 | 2.2 |
+---------------------+-------+-------+
1 row in set (0.00 sec)