Not quite the same issue as the OP. In your case, if your itinerary shows as being booked in Economy, with a corresponding economy fare class, then that's where you'll be.

Try getting the LH locator from your TA and you should be able to access your booking on the LH site to book your seats. You also then be able to see how LH has booked you on their flights.

Edit to add - depending on day/season FRA-ZRH can be operated by CRJs, which don't have J.