I've just had a look at the 3E conversion book, for the first time in probably two decades. It's nonsense!
Eg on p 3, it says "Record the character’s existing ability scores." And on p 8, it says "If you created your character with the 2nd Edition AD&D rules and you used the optional proficiencies rule, you can use your character’s selection of nonweapon proficiencies as a shopping list for skills. Just purchase skills similar to your character’s proficiencies (a list follows)."
In many, perhaps most, cases, following these steps won't produce a character who plays anything like the AD&D character. Just as one obvious example, fighters will have significantly weaker saving throws and non-combat abilities.
A 3E character can be "converted" to 4e by recording their existing ability scores and using their existing build as a guide to their 4e build, too. Voila! A conversion guide!