Hi,
CIFS Server is connected to a SVM and SVM is spreaded to different nodes in a cluster.
Connection from Users are made to CIFS Share which is connected to Volume. and Volume can be on any node within the SVM.
Even though your LIF is on Node1, but if you volume is on Node2, then data is accessed using LIF on Node1 using the backend connection.
You can list CIFS Sessions on each node to know how many sessions are opened or terminate the cifs while upgrade.
Hope this may help.
BR
Raj