How to improve but not change an excel application?

Saw an interesting Excel Application today.

But now have to figure out how to tweak it without changing it too much.
Some parts are really simple..but require some of the code we have done for others..
There are a lot more tricks one can do with the many tabs by having a summary sheet etc.

Was quite impressed how they used the Goal Seek Function - will need to look at how one can code to use the what if functions and add "scenarios" etc....

The watch window is also interesting in that it tells you the current value of a cell or tow - so if you change the value that effects this cell, you can see the value...How this could make it easier for someone or whether we can get these values in code is another story...