I booked a YUL-NCE return ticket with the TATL segment in Z and the CDG-NCE in J. I have already flown the YUL-NCE part and I would like to replace my return flight with a TLS-YUL rather than departing from NCE.

The ticket rules are no change fee only fare difference, and I have also checked that I can depart from a different location within the same country.

However, when I try to make the change online, the difference I am asked for is pretty high (2800CAD at best for 2 persons). I no longer have access to EF, but I assume that this might be due to lack of Z-class availability.

So my question is the following: as I want to depart on the same day and only change the J part of the ticket, will I be able by calling AF to have the agent 'block' the Z part of the flight and only change the first segment? I have done this with AC in the past, but I rarely fly AF and I wonder whether this is a standard practice or not.