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.
Let's answer it from the other side: there is no guarantee that any two versions of the jdk produce the identical byte code. So you can expect differences in general.