Ok. So your parents go to agent (what agent - AF or travel agent) and say that they want to go to YUL on these dates and return on these dates... and agent says .. okay, these dates are available .. total fare is EUR 875.

Yes, it might it is up to the agent to volunteer the information to the buyer, but usually people do want the cheapest fare. So agent probably just assumes that no changes are required. Cheapest fare(for this ticket) is Q class.

(The next fare up is in L class, and that is around EUR 100 more and that ticket can be changed for a EUR 100 per time fee)

For most fares, there are fees to change. The only ones are the full economy class fares for around EUR 1600 per person.

Your parents must also tell agent that they would like a ticket that allows changes... if they don't say anything, then the onus is that they usually want the cheapest fare.

So sorry.. about that. You need to take up the issue with the issuing agency. See if you can buy-up to the next fare class which allows date changes plus the change fee. Ask AF if it is possible for you to do this.

Good luck.