We present a CKKS native homomorphic transciphering framework that converts existing LEA-128 ciphertexts into LEA-256 ciphertexts without exposing plaintext. Motivated by the post-quantum reduction in symmetric key security, our design shows that, by using homomorphic encryption, especially CKKS, it is possible to transition to post-quantum cryptography without revealing sensitive information. Furthermore, we propose a method that optimizes ARX operations by leveraging the SIMD structure of CKKS. Our approach combines: (i) bitwise packing that maps LEA’s ARX to CKKS primitive operations; (ii) a CKKS adapted Kogge–Stone adder that implements modular addition with logarithmic carry depth using masked rotations; and (iii) a low degree noise attenuation polynomial that preserves binary fixed points and suppresses near binary perturbations at a modest depth overhead. With \(2^{15}\) CKKS slots and 32 CPU threads, the pipeline that converts LEA-128 to LEA-256 completes with an amortized time of 6.61 s, and the results decrypt correctly under our parameters. Compared with TFHE style Boolean gate baselines, our circuit formulation and 1-bit encoding deliver over 3 \(\times \) speedups for bootstrapped modular addition and subtraction, while XOR and rotation are substantially faster due to SIMD packing. These results indicate that CKKS based transciphering is practical for ARX ciphers like LEA and provides a confidential migration path from 128-bit to 256-bit keys without plaintext exposure in the post-quantum era.