However I still found it strange, that a mixed cabin was not possible. Maybe that's a relevant info for somebody here in the forum, or maybe somebody was already able to get such a mixed cabin and I just got to an agent who did not know how to do it.
I suspect it's because of the fare rule. You bought a ticket from WAW to ZRH and not a multi city itinerary WAW-CDG and CDG-ZRH.
Doing the change as you wanted would have involved a complete fare recalculation and it may have cost you a lot. Just my 2c...