If you need to use an INT field in MySQL, it's useless to set it to signed values (default) if all the values that you will store there are possitive (eg. auto-increments starting with 1). Also, you will be limited to 2,147,483,647 records.
If you wish to change the field type to unsigned, which will allow you to store 4,294,967,295 records, you can use the following statement:
sql> alter table TABLE_NAME modify FIELD_NAME int unsigned;
Query OK, 0 rows affected (0.88 sec)
Records: 0 Duplicates: 0 Warnings: 0
Read more about INT types here.