The issue here is that the 'Server Objects' - such as Unix Users, Unix Groups, etc are not always backwards compatible. for read operations they will probably be ok, but for deploys there could be issues.
so if you plan on keeping downlevel agents around for some time you may need to make copies of any blpackages, jobs or templates that target the older agents.
if you are going to upgrade your agents pretty quickly after the appserver upgrade then you don't need to worry about this.
this is something we are looking into improving because it is a bit onerous to deal w/ copying things around.