When a new ERP arrives, such as NetSuite or SAP, in-house systems do not disappear on their own. Each one needs an explicit decision.
Retire
The ERP fully covers the function. The in-house system is switched off and its data migrated.
Absorb
The ERP covers the function with adjustments. Rules and data move into the ERP.
Extend
The ERP covers the core and the in-house system keeps what makes it different, integrated with the ERP.
Rebuild
The function does not fit in the ERP. It is rebuilt on current technology from its specification.
How to decide
With a measured sample of the most critical screens: it shows what the ERP covers and what it does not, before committing scope.