Ah.

So you wanted to travel TUN-(CDG-YVR-)YYJ. But because you were limited to AF/KL/DL operated flights, you bought only TUN-YVR.

Then TUN-CDG was cancelled. You didn't like what they offered as alternative so you asked to reticket as TUN-YYJ on a different day, but by the time they repriced it was more than you expected. You bought it anyway? If I read right?

Now YVR-YYJ has been cancelled. And you are worried if you ask then to just keep TUN-CDG they will reprice again and ask to pay the difference?

Technically your route is not subject to EC261/04 protections, but under that you have a right to not take just the cancelled flight and get a partial refund. You can try and invoke Article 8. Otherwise you can argue your rights under Canadian law, but that's not so clear.