Originally Posted by
Flier74
Maybe the booking class was not available anymore the next day when you called again and you would have to be booked into a higher booking class which equals a higher fare difference?
This is most likely. Pricing is dynamic and there are literally thousands of people looking at and then purchasing tickets worldwide at any given moment.
Sometimes it is cheaper to simply let a ticket go and rebook another. But, as others note, if you do this on the outbound, the entire ticket will cancel (unless you are originating in Germany and purchase the higher fares which permit flights out of order).