It took some doing, but we finally convinced management that a live connection wasn't worth the development time and expense--all we had to do was dump the contents of the old system into the new one and be done with it. The catch was that we needed to get to know the old system before we could develop an efficient plan for the new one.
Here are some other lessons learned:
Ask users what's on their wish lists, and do your best to incorporate the features on which there's consensus. Also find out which parts of the current system users think are overkill.
By taking these steps, you'll save time and money on development and implementation as well as post-deployment changes. You'll also earn the respect and support of your users.