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: 
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.
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.