Journalling the files is a good way to achieve what you want although as an alternative you could look at the use of database triggers.
A database trigger can be defined to call a program whenever the physical files are updated (or records created).
There are a lot of examples you can find via Google, search for “database triggers as400”, although a good general page with an example can be found at http://www.tylogix.com/Articles/TriggerTechniques.htm
Hope it helps
All the best