how exactly the import is done? As far as I know you can choose item type during the import.
Changing item type is not easy because each item type is represented by table in the database. If you want to change item type you should delete rows from one table and create new rows in another one. It's not like changing "object_desc" property.
In fact you should reimport items. Or may be in latest versions of Teamcenter there is some utility to do this easily, but I didn't find any in Teamcenter 10.
Changing Item type requires custom ITK. You'll have to map the attributes from one item type to the attribute of the secon item type, same with form data, etc. Naming conventions and naming rules are also involved. It's not easy but many of done it.
NX 11.0.1.mp01 | SW 2016 | TcUA 11.2.3