Section: 4.3.8 Change of Network Slice instance for PDU Sessions
Content: When a Network Slice instance for the existing PDU Session is required to be changed as described in clause 5.15.5.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], the AMF deletes the old NSI ID corresponding to the Network Slice instance that is congested or no longer available and informs the SMF of the PDU Session(s) which is selected by using such old NSI ID to release this PDU session with appropriate cause value as described in clause 4.3.4.2. If so, the SMF triggers the impacted UE(s) to establish new PDU session(s) associated with the same S-NSSAI by using the procedures for PDU Session(s) of SSC mode 2 or SSC mode 3 as defined in clause 4.3.5. When UE initiates PDU Session Establishment procedure, the AMF may select a new Network Slice instance for the given S-NSSAI during PDU Session Establishment by querying the NSSF as described in the clause 4.3.2.2.3. If there is no Network Slice instance available, the network may change the related network slice(s) for the UE as described in clause 5.15.5.2.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2].