Flights in general are slightly overbooked. This is common practice with airlines.
From their homebase there will always be more no show due to missed connections.
If still oversold or to create extra revenue they all offer an Upgrade at a typical rate of 329,- E. FF members can occasionally buy an upgrade on miles in the lounge or onboard.
A free upgrade is very unlikely nowadays. Incase they offer reduced upgrades they make money and get rid of a possible overbooking in economy.
The revenue department does scan all flights for double bookings/fake bookings etc about 48 hrs before departure.