We all encounter users with requirements like this: "This is easy. A college intern set it up in 10 minutes. We just need you to put that functionality in your fancy-pants system." (Well, maybe not that exact wording).
I encountered something like this a while ago. The basic requirement was to generate a little web report in a table. We implemented this in our system, and the user immediately called upset that it wasn't working -- it turned out they copied and pasted from the web page to Excel, and this was no longer working as they expected.
Now I know to ask more questions (and give this dangerous flavor of requirement its proper respect). Some questions to pose:
- Do all the users of this system/functionality use the same system/platform (if I watch how you use the system, is that sufficient)?
- What other systems will use the output of this system?
- How critical is this system? are you willing to live with some hiccups?
- Are you tied to the exact look and feel of the current system?
- How do you currently manage your site, and the content and other data on the site?
In a lot of ways, this is a much scarier requirement than a brand new one. Even if there are misunderstandings in the requirements of a new system, at least everyone understands that this can happen. But a quick ctrl-a ctrl-c ctr-v can be all it takes for a user to prove that your implementation doesn't do what the previous one did.
You should try to sit down and walk through the user actually using the functionality. If at all possible, you should also walk through the system yourself, noting any potential complexities (and discussing this potential complexities with your client). Emphasize that there may be details of the current system that they and we weren't aware of. Hopefully you can also concentrate on improvements that can be made to the system, so they aren't as disappointed with small setbacks.
As to phasing, if possible, try to first deliver a pilot or beta so they can play with it. Also, you need to be ready to "fix" things after delivery.
