79 240d, the outer dash vents have adjustable twist flow control, the inner two dont.

Can the inner two be replaced by adjustables?

How to remove/install the outer adjustables? There is a thin chrome ring and wave spring washer on the outside and the vent assembly behind the dash. Appears to have four small rectangular slots in each piece. I would have expected one with slots and the other with tabs, but no. Seems the mode of failure is these slots break off the edge piece and wont hold together. How are these held together and how do you disassemble without breaking?