In the Plex documentation, please review the following link. https://docops.ca.com/ca-plex/7-2-1/en/release-information/upgrading-from-earlier-releases#UpgradingfromEarlierReleases-UpgradingfromCAPlexr5.5
In the link there is the following section.
RPG Functions Upgrade Requirements
You do not need to regenerate or rebuild existing RPG functions when upgrading to CA Plex r7.2.1.
Plex WinC/WinNTC functions are mandatory to regen/rebuild due to compiler changes. System-I Plex RPG objects are not mandatory to rebuild.
To upgrade to Plex r7.2.1, you need to install Plex r7.2.1 dispatcher on IBM-i and new dispatcher will work fine with existing RPG built objects.
And make sure you have Plex r7.2.1 client runtime (WinC/WinNTC) installed at Plex client application side.