I am not sure.. But my guess is all the topics that is on java 6 and not in java 1.4 ie ( SCJP 6 - SCJP 1.4) Basically he wont have study the topic again for which he has already studied on java 1.4. Otherwise if he goes for the regualar SCJP1.6 he would have study everything including the topics in 1.4.
Hope that helps!
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