Experienced the same thing lately on a CDG-FRA-NRT routing. Was booked in V-class on LH, asked for the upgrade upon check-in in Paris but the only thing the agent could do is put a note on my PNR and told me that I had to check at the gate in FRA. When I arrived at the gat, the upgrade had been cleared and had an excellent seat, window exit row. On the flight back, I got upgraded upon check-in at NRT, but only till FRA, upgrading to CDG would have required another certificate, of course not worth it.