This depends on the nature of such a change.
In most cases, IB makes new API versions down-compatible with previous ones.
But if something principal will be changed, then DO won't work with such changes of API.
It will be great for the existing customers if some software company will decide to obtain the source code and a user database from us - in that case, they will be able to modify the program in the API change situation. Bearing that in mind, I do not ask really big price for the source code. There are couple of conversations taking place now, but it's really too early to promise something - and there were several requests like "I'm a good programmer, but I can give you only $300(500, whatever...) for the source code and I promise I will take all of your customers" - this is of course, not acceptable (I'd prefer to make it open source in that case, but it contains several pieces of 3rd party info that can't be publicly disclosured, so this does not gonna happen).
Another possible soultion would be a sell of the source code to some self-organized body of traders/customers which would be able to hire contractor programmers to modify a code in a case of need (just an idea, but it should originate from the other part of the table, not from me).
Some off-topic (after reading other Refco-related posts at ET): I was at one of the Refco Toronto divisions today (...yesterday, sorry) and did not notice anything unusual - people taking orders, speaking with clients, computers blinking, the floor is clean and shiny...