One case is examined in detail to determine value. To keep cost of the survey down, we sacrifice statistical validity for concreteness.
The detailed impact case method is good for tools and methods changes that have a large effect either because they are used on one big project or because they are used with small effect by a great many people, all of whom realize more or less identical benefits.
It is also good for cultural changes, for the same reasons.
Any time someone mentions a changed interaction with another person; you must trace it down and interview that person for effects of that interaction.
For example, you must follow all changed documents through their life cycle.
What seems trivial to one person may have an enormous effect on another.
Even with each person, several iterations and much imagination may be needed before all effects are listed.
Even though you are using one case, you may want to brainstorm with or interview several people who are affected.
That way, you can develop a laundry list that includes all possible effects. This list can be reused in similar studies later.
Once you have the list of impacts, you must attach a value to each.This again may take great ingenuity.
You can limit the amount of work by guessing which are the big ticket items and doing those first.
Note:Stop when you have enough to be convincing, rather than perfect