For #Ethereum's #ProgPow transition, I do think it might be possible to allow ASIC and GPU to co-exist, while making sure ASIC does not have advantage. (1/2)

Allow both #ProgPow and #Ethash seals in a block. They share difficulty adjustment. Ethash's actual required difficulty is multiplied by the ASIC efficiency factor. With this setup, ASIC continues to mine with Ethash, and GPU mines with ProgPow. No one has advantage. (2/2)

In #Ethereum, a switch of mining algorithm, no matter to #ProgPow or #Keccak256 will to some extend break backward compatibility. This is because EVM allows access to past block hashes. Whether this breakage practically matters is a different issue. Maybe, maybe not.