Pega does not allow to modify the keys of a class if it have some existign work objects in DB, give a try as per below procedure.
1. Copy all the instances to a another temporary table in DB manually.
2. Truncate the original table.
3. Now change the keys to the class form.
4. Put back all the records to the original table.
5. Obj-Browse all the records with complete BLOB data on the clipboard page.
6. Loop through all the pages and do Obj-Save with commit.
7. So new records are generated to the DB with new keys.
8. Delete the old records from the data base by using like query on pzInsKey column.