While it would be possible to run old Plex applications in newer System i/Windows versions, it is possible issues could exists. Indeed, my first question is about your Plex version, and the possibility of upgrading, but I'm assuming it is not possible for you,
I used to run sometimes two Plex versions with different dispatchers and library lists, in a case, really old. Theoretically it is possible, but you need to be aware on issues: it could require patches on the server side. You should attend messages from the communications process. There are some troubleshooting suggestions on Plex guide for System i Version.
Having changed your Sistem i installation, you should review system i Plex requirements for runtime objects, library list, user profiles, communications setup. It is possible some of them have been changed. It is difficult to be more detailed in this case.