It is possible to change an item type's field that is already in use by using the Roundtrip export.
To start, you need to create a new data field. Go to Admin > Item Types > Select the item type > Fields > Add Field to create the new field.
Note that the new field should be of a new type. This will allow you to change the field type, by replacement.
You also need to take into consideration the type of data you have when changing the field type. For instance, text content cannot be stored in an Integer as an Integer can store only numbers.
After creating the new field, you need to add it to the List View so it can be exported. You can do this through the Admin > Item Type > Configure View option, by adding the field to the list on the right.
Alternatively, you can use the Configure View option in the List View to do that.
Locate the items you want to change by creating a filter or opening their set in List View. From the List View, open the Export Menu > Office Templates.
Select and Run the Excel Export for Roundtrip.
This will generate an Excel export. Open the export in Excel and copy and paste the data from the old field's column to the new field's column.
You can now save the spreadsheet and import it back into Jama Software.
Also, don't forget to check the box for round-trip during import.
All items within the Export/Import will be updated, regardless of which Project you upload them into, so this is a good way to update items across Projects if necessary.
If the new field type is compatible with the data imported, you should be able to see identical data in the two field types (otherwise, the new field will be empty).
After you confirm that the data was successfully imported to the new field type, you just need to go to Admin > Item Types > Config again and delete the old field to complete the change.