Method and apparatus of supporting lossless pdcp version change in next-generation mobile communication system

A terminal in a wireless communication system, the terminal comprising:

• a transceiver;
• and
• a controller coupled with the transceiver and configured to:
• receive, from a base station, a radio resource control (RRC) message including information indicating a change from a first packet data convergence protocol (PDCP) entity for a bearer to a second PDCP entity for the bearer, the first PDCP entity and the second PDCP entity being in the terminal,
• identify whether the first PDCP entity for the bearer is changed to the second PDCP entity for the bearer based on the information indicating the change,
• process data with a PDCP sequence number for the first PDCP entity as new data for the second PDCP entity in case that the first PDCP entity for the bearer is changed to the second PDCP entity for the bearer,
• release the first PDCP entity,
• configure a new PDCP header associated with the data, and
• encrypt the data with a new security key,
• wherein the PDCP sequence number of the first PDCP entity is greater than or equal the PDCP sequence number of data for which successful transmission is not acknowledged from a radio link control (RLC) entity.