With a roundtrip award both legs would be rebooked (and therefore repriced) if you make a change (or cancel) the return prior to flying. Therefore it is always recommended to fly the outbound first, and then call in to cancel the return. As the outbound has been fully flown that won't be repriced.

However in this case we are talking about a single one-way with a transfer point in between. I don't think they will allow you to change the one-way into a stopover at that point halfway, as Flying Blue does not offer any kind of changes (They only allow cancelling & rebooking). But even if they would allow that change (Which I doubt), I believe it would be a very painful exercise to find an agent able to actually do it. Even booking a regular stopover award can be quite painful to find a capable agent. And this will be on another level regarding complexity.

I would recommend either one of two options:

- At CDG call in and request the ticket to be adjusted (Cancelling the ZRH leg). They may charge you a fee (in miles, hopefully using historical fares) as the long haul may be more expensive to CDG compared to ZRH. Additionally you should be entitled to a tax refund (No CDG transfer tax should be charged).
- Just miss the ZRH flight and optionally call in afterwards to request a tax credit. However if the transfer taxes are low I'm not sure if I'd personally open that can of worms at that point. They may try to reprice if you do.

Both would require a separate flight to ZRH a few days later.