Answer
Yes — change internal IFS system-user passwords through the IFS Installer: run the installer on the Main node and choose Reconfiguration, then use the Edit option on the internal System Users page. There are two distinct sub-options there: changing only the middleware configuration files' stored password reference, if the underlying Oracle-side password itself isn't actually being changed, or changing both the actual Oracle database user's password AND the middleware configuration files together, if the Oracle password itself does need to change. In a clustered deployment, remember that running Reconfiguration this way only directly updates the Main node — you'll need to propagate the resulting configuration out to the rest of the cluster nodes afterward, the same way you would after applying any other delivery.