A first base station (BS), comprising: at least one storage device, configured to store program codes; a first communication interfacing device, configured to transmit signals or to receive signals with at least one BS; a second communication interfacing device, configured to transmit signals or to receive signals with a plurality of communication devices; at least one processing circuit, configured to execute the program codes comprising following steps: (a) associating a first PDCP SDU to a first PDCP SN or associating an IP packet included in the first PDCP SDU, wherein the first PDCP SDU comprises a flow ID and an IP packet; (b) initiating a SCG change for a first communication device with a second BS; and (c) controlling the first communication interfacing device to forward the IP packet but not to forward the flow ID, to the second BS in response to the SCG change.