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,
• transfer first data from the first PDCP entity to 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 first data, and
• encrypt the first data with a new security key,
• wherein the first data is associated with a first PDCP sequence number which is greater than or equal to a second PDCP sequence number for which successful transmission is not acknowledged, from a radio link control (RLC) entity.