If you buy Lufthansa tickets with return are considerably cheaper from one way. Lets say I want to book from A->B and back. I am not sure yet about the return date. They are offering Economy Flex in which you can re-book portion of the flight. My question is what price will the new ticket have? will the price be calculated as a return flight or one way? Because the same ticket one way costs 300 and if it is a return ticket 300 in total.
The rules are typically as follows:
When you change the return flight you have to pay the difference between the cost of original return flight and the cost of the new return flight plus any change fees. Cost difference between the tickets goes both ways: if your new return flight is cheaper, you actually get money back.
If you buy a flex ticket, the change fee is normally $0. However you are still responsible for any difference (positive or negative) of the actual ticket prices. If, at your new date, only very expensive tickets are available, you may have to pay a lot extra. In any case you should read the fine print on the flex ticket, sometimes there are exclusion of when and how the change fee is waived.
In the end this is simple math: A flex ticket only makes sense if the price adder for "flex" is considerably lower than the change fee for a regular ticket. In practice, that's rarely the case and typically flex ticket don't make a lot of sense.