I'm not sure if you're going to have any success with this.
I believe the inserts are done first for a reason, as the opposite has the potential for violating integrity constraints in certain scenarios.
You might be able to remove the object from the Hibernate Session through an evict, which would stop Hibernate from managing the persistence and doing the inserts first. Then, you could perhaps just do an actual update, or a delete and then an update (although that could be going a little too far.)
For a little bit of background on why Hibernate might be performing those updates, take a look at this little tutorial that goes into How Hibernate Works, and why Hibernate does the things it does.
How Hibernate Works: Understanding the Hibernate ORM Model and Mechanism
Happy Hibernating!