The BSS stores the Kc and forwards the Set Cipher Mode command to the MS. The CIPH_MOD_CMD only tells the MS which encryption to use (A5/X), no other information is included.
The BSS preserves the Kc and forwards the command to set the cipher mode to the MS. The CIPH_MOD_CMD only specifies the encryption protocol (A5/X) for the MS, without providing supplementary data.