1. on two pages he had written something different than what appeared on the map illustration,
2. he notes that there were several inaccuracies, and
3. he had just published The Lord of the Rings

What this last thing has to do with anything is that the original has Gollum, while still being a wretched creature, as a nice guy who wants to help Bilbo. After writing his longer work, he noticed that he had really beefed up the evilness of the ring and its effects on Gollum, so he made several changes. In his introduction to the book, he explains the major changes to Chapter 5 this way (keep in mind that he pretends to only be a translator of ancient texts he found, not the true author - Bilbo is the supposedly the true author of The Hobbit):

... More important is the matter of Chapter Five. There the true story of teh ending of the Riddle Games, as it was eventually revealed (under pressure) by Bilbo to Gandalf, is now given according to the Red Book, in place of the version Bilbo first gave to his friends, and actually set down in his diary. This departure from the truth on the part of a most honest hobbit was a portent of great significance. It does not, however, concern the present story, and those who in this edition make their first acquaintance with hobbit-lore need not trouble about it....

Here are our reading texts for this unit:

• Riddles in the Dark - the text from the chapter
• Riddles in the Dark - as read by J. R. R. Tolkien
• Riddles in the Dark - The Lost Version: side by side comparison of original chapter with revised chapter
• Mind's Eye - Beyond Just Listening - a radio dramatization of this chapter

Before the movie came, there were various opinions of what Gollum looked like:

Trailer for the 2012 Hobbit part I - an Unexpected Journey

Trailer for the 2013 Hobbit part II - The Desolation of Smaug