Caution: Changing a field type modifies all the data in a
field (column) so that it conforms to the new field type. If the data
cannot be converted, it is lost. To prevent the loss of data, duplicate
the field that you want to change, and then make changes to the duplicate.
To change a field type:
Click on the Change Type link above the Properties tab.
Note: You cannot change the field type of a file attachment field.
From the Select a field type drop-down list, choose the new field type, and then click Convert Data.
Note: You can't change the field type of a list-user field. In addition, you cannot change another field to a list-user field.