You need to clear user recent Fiori Launchpad (FLP) activities, because of changes in the Target Mapping, which would lead to an SAP standard error dialog, when trying to open the obsolete app etc.)
Like other configuration and personalization information of the FLP, this is also stored in WebDynpro setting tables. There is a dedicated transaction to reset such data. The thing you need to know is the name of the personalization container to wipe.
Start transaction /UI2/PERS_DEL and enter sap.ushell.services.UserRecents
The result list will show you a line for each user who has recent activity shown in FLP, which was deleted. U means user scope/personalization.
That was an operation you can execute for individual users, or in mass in SAP GUI.
Of course the users can do the same from the Fiori Launchpad, just open your settings with the Settings button shown on the first picture, then push the Clear button, or even turn off history with the switch under User Activities.