This is one of the risks that you take when you hold a reservation and choose to delay ticketing. Prior to ticketing -- any changes are your problem (and risk). After ticketing, any changes are the airline's problem. If you had already been ticketed, then IB would have reaccomodated you automatically (albeit in the inimitable surly IB style). They aren't obliged to put you on the best possible alternative, but to offer some alternative (which may even be to fly another airline). Next time get the ticket issued and then you will be protected.