> I don't see it as a big problem to change plans
> somewhat at the start of a project, this is certainly
> better than doing it half way through.
There's a lot hiding behind that word "somewhat".
In his book Software Estimation: Demystifying the Black Art [1] Steve McConnell says that most project schedules can tolerate around a 20% change in delivery time without too much distress. So if the "somewhat" is within the manageable +/-20%, then the project can move forward. If the change is bigger than that, it almost certainly means that the project deliverables, schedule and fees need to be renegotiated. And if a client is constantly causing a project to be renegotiated, are they really a client?
In his book Software Estimation: Demystifying the Black Art [1] Steve McConnell says that most project schedules can tolerate around a 20% change in delivery time without too much distress. So if the "somewhat" is within the manageable +/-20%, then the project can move forward. If the change is bigger than that, it almost certainly means that the project deliverables, schedule and fees need to be renegotiated. And if a client is constantly causing a project to be renegotiated, are they really a client?
[1] http://www.stevemcconnell.com/est.htm