A subscriptions to Expert Flyer will let you know which flight have upgrades available at the time of booking. Often you need to be waitlisted, and that requires a call, but you would do that AFTER buying the ticket. Expert Flyer has a 5 day free trial.

For a change, consider flying to CDG, then taking TGV to Marseille
The shorter trip looks to be around 3 hours 21 minutes.