So, you might want to slow down a tad on that intellectual shift away from the Java 6 certification to the Java 7 Certification. Oracle has made some pretty major changes to the way the certified professional designation is obtained, not the least of which is the fact that the OCAJP (the Oracle Certified Assosicate: Java Programmer) designation is a prerequisite. So, you now have to pass two exams to get the professional designation, and with exams being $300 or so a pop, you're looking at significantly lightening your wallet. Of course, if you do go the Java 7 route, you end up with two designations, not just one, and you get your credentials on the latest version of the JDK, so there is some justification for passing up on Java 6.
As for what is new with Java 7, the best place to go for that is the objectives page for the Java SE 7 Upgrade exam. Basically, it tells you everything that's no the new exam that wasn't on the old exam, or as the name implies, it's what you need to know to be qualified on JDK 1.7. What are the new topics? Well, language enhancements are tested, which include things such as switching on Strings, using binary notation, working with underscores in numeric literals, and the good old try-with resources enhancements. Also, some new APIs are examined, with JDBC 4.1, Java File IO enhancements (NIO.2), concurrency programming and localization.
One new topic that is being tested on that I thought was interesting was Design Patterns, namely the singleton, factory and DAO patterns. That's a nice switch to say the least.
Check out the objectives for the upgrade exam, and you'll see what the key differences are between what you are studying for Java6, and what you'll need to know to get Java7 certified.