You can only change a dictionary entry's data type when the change does not result in data loss. Use the following guidelines to change a dictionary entry's data type.

Table 1. Valid data type changes

Condition	Restrictions
The field is empty in all table records.	None. You can convert an empty field from any data type to another without restriction.
The table contains existing data for the field.	You can only convert between logical data types that map to the same physical data type in the database. For example, you can convert a glide duration to a glide datetime since both logical data types map to the DATETIME physical data type in the database.
The field is a string field you are converting to another type of string field.	You can change between string-based data types as long as length changes do not cause any data loss from truncation. For example, you can change from a MEDIUM database type to a VARCHAR(100) database type if none of the existing data is greater than 100.
The field is a string field you are converting to a globally unique ID (GUID).	You can only convert a string field to a GUID if all of the exiting data in the field are already Sys ID values.
The field is a GUID field you are converting to a string field.	None. You can convert a GUID field to a string field without restriction.