Once CU makes the decision to start handover procedure : 

• Optionally the gNB-CU may send an UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU to query the latest configuration. The source gNB-DU sends the UE Reconfiguration Request to MAC/RLC layer and get the UE Reconfiguration Response from the respective protocol layers.
• The source gNB-DU responds with an UE CONTEXT MODIFICATION RESPONSE message that includes latest full configuration information.