Hi Businesstraveller,
There is no way of knowing which 747 fit you will get. New F (Prime) is only guaranteed on certain routes. However on routes where this is not guaranteed you could get a New F or old F bird. It really does depend on the day.
On a positive note, any retired 747s with the new F product on will have that removed and refitted onto aircraft with the old F cabin still fitted. At least your chances will be improved somewhat!
The old F isn't 'bad' per se, remember you get all the lounge things in addition to the service, food and drinks. The experience certainly won't be a disappointment!