1) What is the primary purpose for the Custom House introduction?

The purpose for the Custom House is to basically prepare you for the book. Nathaniel Hawthorne wants to bring you back in the day to know what life was like to better yourself, the reader, for what you will read later. It is also an introduction into Hawthorne's thought process. He is a very intelligent writer and here's your introduction into that.

2) What significant change does Hawthorne admit to making in telling the story contained in the diary?

Hawthorne admits to not wanting to actually tell it like he did. With the way he told it he gave a way to much information. But at the same time it was enough to make the reader understand what was needed. This will have a significant change on the book as a whole.

3) What potential impact will this change have on the story?

This incident will effect how we will look at the story. We now have an inside scoop as to what the book might contain. You will be looking for something that will not happen. And this will hit the reader by such a surprise that it will attract you to the book.