Yes you can’t use a second class pass in first. You’ll have to get it changed.

@cdwatkins19 is right. As your train was cancelled though, they're obliged to offer you an alternative. Perhaps there were no 2nd class available. 

If there's time go back to the service centre and get the reservation changed from 1st to 2nd. If they can they will. If they can't, ask for a note or something "class upgrade due to cancellation" you can show to the train staff. 

If there's no time left, talk to the train staff before and after you board to explain you're happy sitting in 2nd class and due to a cancellation the reservation got changed. 

Unfortunately my experience of the French rail staff during this strike was hit and miss and some are not at all accommodating. So make sure you properly add and activate the right journey (date and time) in your trip in the app so they can't find any fault with Pass itself. 

Let us know how you get on.