Provided is a method for changing, by a user equipment (UE), packet data convergence protocol (PDCP) version. The method may include: receiving a security mode command message, which includes a first security algorithm configuration for a PDCP of a first system and a second security algorithm configuration for a PDCP of a second system, from a base station (BS); deriving a first security key for the PDCP of the first system, based on the first security algorithm configuration; when the security mode command message passes an integrity protection check based on the first security key, changing the PDCP version from the PDCP of the first system to the PDCP of the second system; deriving a second security key for the PDCP of the second system, based on the second security algorithm configuration; and transmitting a security mode complete message, based on the second security key, to the BS.