The publisher, Knopf (a division of Random House), is offering a free replacement of the first printing of Sibley’s Guide with the much improved second printing:
If you bought the 1st printing of Sibley’s 2nd edition then Random House will send you the improved 2nd printing if you call Random House customer support at (800) 793-2665.
Thanks to Dalcio on BirdForum for pointing this out.