Well speaking of the conversion guide, let's go into detail about what is actually in there (edit -- leave it to Curt to beat me to it!

):
Its official title is the "free D&D v3.5 Accessory Update" and it's true to its word -- it is mostly about providing updated information on several of the non-core books. Here's what's inside...
Page 1 -- cover
Page 2 -- standard back of title page info
Page 3 -- TOC and brief essay: "The Nature of the Core Rulebooks' Changes"
Pages 4-7 -- Player's Handbook Changes: Summaries of the changes to races, classes, skills & feats; spell changes including charts showing new spells, deleted spells, spell name changes, spell school changes, spells with substantial changes, and spell level changes.
Page 7 -- DMG changes: a very brief summary.
Pages 7-8 -- MM changes: a brief summary, and then a short essay: "Behind the Curtain: Damage Reduction" with guidelines on how to assign the new method to existing creatures.
Pages 9-12 -- Deities & Demigods changes: how class changes affect god stats, how MM changes affect god stats, changes to divine abilities, domains, and spells; and changes to the monsters/creatures in that book.
Pages 12-23 -- Epic Level Handbook: provides epic info for the new DMG PrCs; Epic skills & feats (changes); changes to epic magic items; and changes to epic monsters. In cases where the feat or item changed substantially, the entire feat is printed. Monsters in this case are provided in a table listing name, type, space/reach, DR if any, feats, and other notes (including level adjustment) if necessary.
Pages 24-25 -- Fiend Folio: as this was basically 3.5 compatible there's not much to convert. There's a list of every monster with DR info; an update to the info on constructing the various Constructs in this book; and an update to the Swarm subtype.
Pages 26-29 -- Manual of the Planes: brief updates to PrCs, spells, templates, petitioners, and monsters.
Pages 30-37 -- Monster Manual II: basically a mini-stat block for each monster with name, type, space/reach, DR, skills, feats, level adjustment, and notes.
Pages 38-39 -- revised character sheet
Page 40 -- advertisement for the Miniatures Handbook
By far the biggest bulk of the conversion info is simply revised monster stats, followed by the charts listing the changes to spells in the PHB.