AF/KL is quite particular regarding booking class availability. Example of AMS CDG 16JUN 6:45am

J6 C6 D6 I0 Z0 O0 Y9 B9 M9 P9 U0 F0 K0 W9 H0 S9 L0 A9 Q9 T0 E9 N0 R0 V0 X0 G0

One way fares are usually using booking class T L H K F U P, so here only P available for one way before the very high Y B M fares. These are also the fares that have no minimum stay requirements at destination, for short business trips midweek typically, when using them for round trips.

But other buckets could also be used one way, usually tied to advanced purchase requirements, like G X V R N E Q A S W, depending on originating and destination markets. For round trips, these fares usually have weekend stay and/or 3 days minimum stay requirements.

If you continue after CDG on AF, like AMS CDG WAW, for one way there are G X V R N fares with 15 advanced purchase and then E Q A S W fares without advanced purchase, so AMS CDG WAW using the above flight is likely to be cheaper (E class) than AMS CDG in P class.

Here, it could be that the original ticket was booked let's say in U class, which is higher than Q class, there is a Q fare that qualifies on the new flights with Q availability whereas U is not available (like in the example above), therefore changing the ticket would require an upgrade to the next available fare, like P with a fare difference being collected, whereas a cancel and rebook into Q class would save money (and you lend money interest free on 2 tickets at the same time to AFKL until the refund of the original ticket is processed).