The only thing that is tricky for me is the fact that you're on z/OS.
By changing the retention inside the Application Group definition, you change the retention for ALL documents loaded into that AG, starting at the time the change is made. (Obviously, you can't get documents back that have already been expired.) The only snag is that if you're using secondary storage (TSM/OAM) that you need to make sure the change is made there as well.
Here's a scenario I see all the time:
Retention is extended from 3 years to 7 years in the CMOD client. However, TSM was set up to expire the documents after 3 years, and wasn't updated -- which means that TSM starts deleting anything that was loaded 3 years+1 day ago. So CMOD keeps the metadata, and queries come back fine -- but anything older than 3 years fails on retrieval. The copygroups and management classes need to be updated in TSM as well.
There's another caveat as well. In a lot of sites, Application Groups use a small number of storage sets - often organized by retention period. If your AG belongs to a 3 year storage set, and you increase the retention of that storage set in TSM to 7 years, then ALL of your documents that were scheduled to be kept for 3 years will now be kept for 7 -- which is arguably even worse.
Depending on how complex your scenario is, you might just want to extract and reload those PDFs into a properly defined Application Group.
-JD.