Rebuilding software applications is something I've thought a lot about in the last 18 months.
Rebuilds are special and have special considerations.
I wrote a blog about it for work you can read here which talks about some obvious pitfalls.
Here is another thought - what people like about their old system is actually more important than what they don't like.
This may seem obvious but here is the hard part: they are not going to tell you the things that they like. They are going to spend their time telling you what they hate about their current system.
This can happen for a number of reasons. It can happen because complaining is fun. It can happen because it is easy to criticize. And, it can happen because people do not want to be allied with the old regime once the winds of change have begun to blow.
Often, hated features are simply old and would have been corrected by updating the system.
You have to be careful and look hard for what they love, what was great, and what they have to have.
And here is the biggest catch of all. Sometimes people love features that they do not know they love because those features are so well done that they don't even realize that those features are there.
How will you know they love them? How will you be able to recreate or improve upon a feature that is so great that it is barely perceptible?