@vesuvius: The documents have been thoroughly vetted and pored over by the folks doing the analysis. These haven't been static documents. We've also got approximately a dozen other projects going at the same time, so not all the time was on one project.
Under threat of it not getting done at all, the customer has been very engaged. The analysis team (lol, it was two people) knew that it would take a long time to complete the analysis because of other commitments, so at least once a month, they went to key stakeholders and refreshed the user stories and other findings, as well as validated any assumptions.
As far as change goes, that's been sticky a few times, but I think we've got the process hammered out. Like you indicate, the trick is to expect change. We're also trying out a more agile process with smaller sprints and regular deliverables, including cycles for UAT and minor adjustments, so that's helping with managing change and expectations.
That said, yes, you're concerns are very much on everyone's mind, and we're very cautious.