I made many changes to update and improve the story.

I’ll start with what I didn’t change. The plot of the book starts exactly the same, with Grover trying to find a cure for his impotence. And it ends the same way (which I don’t want to spoil if you’re reading these answers before reading the book for the first time). The book also has the same locations, plot points, and characters (albeit some have new names).

When it came to changes, I made adjustments to the language, humor, pacing, themes, and character development. You have to understand that I wrote the book in 1989 and did only one revision before it was accepted for publication by Ace Science Fiction and Fantasy. The editors improved that draft with their feedback and copyedits. However, it was still a second draft that got published. Sentences needed to be fixed for flow. Some jokes weren’t funny (or just plain offensive). And sometimes, a scene would wander and needed to be trimmed to keep the story moving. 

Finally, the themes and character development needed to be brought out more. When I wrote Heroes, Inc., I hadn’t planned on writing a fantasy series. So when I revised the three novels, I wanted to ensure the themes were consistent from book to book and show Grover and Cilla evolving as heroes and in their partnership.

I understand if fans of the original editions might not enjoy the changes. However, they can still find old print copies if they want to enjoy the original versions.