The changes introduced by this EIP are applied beginning with TRANSITION_POS_BLOCK (the first PoS block in the network). Beginning with TRANSITION_POS_BLOCK, the DIFFICULTY opcode will start to return 0 as per the deprecation of the difficulty field introduced by EIP-3675. The idea of preserving the DIFFICULTY instruction number for the new RANDOM opcode is in removing backwards incompatible change introduced by EIP-3675 with respect to existing smart contracts. Existing smart contracts will be able to use the EVM instruction with the same number as a source of randomness outputs disregarding the Merge has happened or not. Although, they will have to take in account the change in the size and the nature of these outputs post-Merge as it’s stated in the EIP.