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: 
The bytecode could slightly differ, but this is nothing to worry about since it will be still compatible. 

What really will be executed depends on JIT.
If you compile with different JDK versions I would advice to use the *target* option to javac.
Otherwise you may not be able to run the jar with an older JDK.

You may also want to use the *source* option to javac, in order to make sure the developer do not use classes/methods added in a more recent JDK.