Hi,
If I remember correctly. as was explained to me some time ago, the class that you are booked on will depend on availability on that particular flights. If my understanding is right, the airlines only assign a certain number of seats per class on each flight.

I have used Ticket Travel Agency based in Mallorca for all my seaman travel bookings - contact them and they are sure to put you right. The link to their seaman fares or Marine fares as they call it, is at http://www.ticket-es.com/Marine-Travel

I hope this helps.