If you set things up right, the old process is upgraded to use the new code upon processing the next message (and has an opportunity to upgrade its state from the state the old code was using to the state the new code was using, if required); like, the idea is that you are trying to upgrade the running code. (In fact, leaving old processes lying around is actually problematic as you can only have two versions of any module loaded, the obsolete one and the current one; if you upgrade again all processes running the old version crash.)