My understanding is as follows. There are two things that must happen before you have a binding contract:

• You have to pay for the ticket
• You have to fly the first sector

This is not only for RTW tickets, but for all international tickets where the carriers use the common fare tariffs. So it is possible for the airline to change the fare after the ticket is purchased and before the first flight has been taken, but in practice that is bad business and they do not. If they did, the ticket holder may elect a full refund with no service or cancelation fees etc.

Also note that the tariff calls for a ticketed destination, not a reservation. This is where an open sector remains protected. If I have an open dated ticket for a destination, and I have flown the first sector, the carrier has a legal obligation to get me to that destination at any time when the ticket is valid and used in the correct sequence. For this reason, some airlines will not issue an open dated coupon for a seasonal route as they may wear an exposure under tariff rule 80 or 90 if they do. Sometime you can get such a ticket issued by an agent who does not understand the implication of the exposure to the airline of tariff rule 80 or 90.

Normally a flight cancellation would be handled under tariff rule 240, where the passenger fails to get to the ticketed destination through no fault of their own (mis-connection, cancellation, change of aircraft resulting in insufficient seats, oversold etc). This, however, assumes the carrier still flies the route. If they no longer fly the route, but still serve the destination port (or someone else does), tariff rule 80 applies.

Rule 80 provides the carrier with essentially 3 choices.

• Re-route via another routing to the missing point on their own
  services without charge
• Re-route via another carrier to the missing point, either directly
  or indirectly, again without charge to the ticket holder
• Alternative transportation to the pont that is acceptable to the
  customer. That might be as simple as simply paying for a hire car, or
  as expensive as chartering an aircraft.

There are some cases where the missing port is an intermediate stop, the carrier can simply drop it without doing anything. This depends on the carrier and their specific carriage rules. For example, on discounted economy tickets
only on QF, they do not need to carry via the missing intermediate port (connection without stopover), but just get you the next stopover point. On QF if your ticket is Y or higher, they cannot just skip the missing intermediate port. This varies somewhat from carrier to carrier.

If they elect not to do any of the things listed above, tariff rule 90 comes into play. Tariff rule 90 is an involuntary refund, and any time the word involuntary is used, it is bad news for someone. Two reasons it is bad news - firstly it is going to be expensive, and secondly it is real money (not vouchers, coupons etc).

The amount of refund has nothing to do with the value of the ticket either. It is calculated as half of the unrestricted return fare in the class of service the ticket entitles the customer to travel in. So if the ticket was say a business class frequent flyer ticket, the refrund isn't the return of points/miles, it is the payment of an amount equal to half of the unrestricted return in business class. If the missing sector was part of the Round the world itinerary, the refund even for a relatively modest sector, could be sizeable portion of the total fare.

So, once you have flown the first sector of your RTW itinerary, even with an open dated coupon the airline has a responsibility to get you to the destination stated on the coupon, provide it is done within the validity time of the ticket and the coupon is presented in the correct sequence. In the case of MAN-AMS example above, I would expect BA would chose to re-route you at no cost to you via London. If there is a reluctance to do this, then mentioning tariff rule 80 and 90 will likely get some action .

You need to ensure you are not taking a voluntary re-route (ie call the airline and ask to be changed to a particular routing) or they can chage you for the re-issue and it must fit inside the fare rules, such as max 20 sectors and max sectors within a continent, OW carriers etc. If the re-route is involuntary, they do not need to re-issue and the invol re-route does not need to comply with the fare rules. They just have to get you from MAN to AMS within the validity of the fare dates and in the ticketed sequence.