Huh, they seem to have removed the introduction and example codes that used to be available on the iText site. I guess that means they *really* want folks to buy the book; not a particularly nice move (I very much liked the first edition of the book, though, it was worthwhile having for anyone using iText seriously). Nonetheless, they'll lose users over this.
Out of curiosity, why do you want to upgrade? Can iText 5 do things that iText 2 can't?
Also note that, according to iText's main developer, iText 5 is not yet stable, and the API might change well into the summer of this year. In other words, the time to make the switch has not yet come.