What must be true for you to change field types and constraints in a table?

To successfully change field types and constraints in a table, it is essential that the table has 0 rows. This requirement is in place because modifying the structure of the table, such as changing field types or constraints, would conflict with existing data if any rows were present. If there were rows in the table, the changes could potentially lead to data integrity issues or loss of data compatibility, as the existing data might not align with the new field definitions.

When a table is empty, you can make structural modifications freely without worrying about the existing data, ensuring that any changes applied would be applicable and safe for future rows that will be added to the table. Therefore, having 0 rows allows for a clean slate to implement the desired modifications.