For one reason or another it is decided to change from the existing architecture and reinstal on another base. For example, a move from Mainframe to Unix or NT.
I know of one case where the Mainframe will be replaced by Windows NT running SAP R3.
Another company is replacing an IBM 3083 with a Unix system running an MRP package.
Essentially a restart from scratch. Requires retraining of staff, and possibly redefinition and reanalysis of computing requirements. In some cases Migration of Data is a major issue. Assigned "Object Class 8".