Expect to pay 500 EUR round trip for Paris Nice at abonné fare, I don't think it is worth it in most cases if you usually buy basic.

For the CDG ZRH, it depends on the reason why your plans change. If it means to finish a meeting earlier, the FLEX fare (or business) is really advantageous since you can go show on an earlier flight on the same day. If you want to change to a later flight or to a different day and take this flight on short notice, you need to pay fare difference, and you need to be lucky to find 100 EUR sectors with little or no anticipation. On the other hand, you can also postpone the sector to much later when the same booking class is available and get back to ZRH as you are doing now.