I have a ASP web application working with SQL Server. It has around 500 records and it works great. Now my management wants to redo the application for the 5th time in the last several months. The management knows nothing about web development and engineering.

The management now wants to modify the site to the point where current fields and tables will not be able to handle the change.
There are alot of things they want that can be done but there are also alot of new things that will effect the current data structure and data integrity.

Any user friendly words I can tell them or experiences anyone has had with this in the past?

All this means is that you have more up-front planning to do.

You must map the current db structure to a new db structure.

This all takes time of course, and it's your job to communicate to management the extra time involved. To help illustrate your point, make a visual representation of what could happen to the data if this important step is missed.

Given enough time, almost anything can be accomplished. You must make management either give you that time, or decide that the time investment isn't worth the desired outcome of the changes.