Question: A hypothetical scenario:

I've got a project whose source compliance level is specified to 1.5.
Now I compile this project with two different JDKs: At first with JDK 6 Update 7 and then with JDK 6 Update 20.

Do these two different JDKs produce different Java byte code, although they only differ in their Update version?

Answer: 
There is nothing that would stop the different versions from generating different bytecodes, as long as it's compliant to the *behavior* specified in the JLS. The JLS leaves many implementation details to vary from one implementation to another.
Why on Earth would someone go to the trouble of issuing an Update of a development kit if it didn't lead to changed byte code in at least some cases? I strongly doubt anyone would do it just for documentation updates.