I think a hardfork change is too drastic, and will certainly end in a contentious hard fork. A POW change light can be implemented as a soft fork by a requirement for an extra proof of work of a different type in the coinbase transaction or in another special transaction. This will encourage cooperation between miners having lots of specialized SHA256 hardware and users mining the extra proof of work on their CPUs.
Good thoughts but miners will never approve this proposal with BIP 9 and I doubt even 51% so would need to be a UASF , whicj will likely end up as a HF only . This proposal is more of a HF in reaction to a 51% attack from miners which would not be as controversial.
The current miners will still have a huge advantage with the extra-POW soft-fork model, since SHA256 hashing power as well is required to find blocks, so I think a large enough economic majority will make the current miners come along in a UASF. The miners have no interest in mining worthless coins after all. They will have to share their power and some of their income with CPU miners, since none of them can operate alone, but will likely still have most of the payout. It is easier to recruit another CPU miner for peanuts, than getting enough ASIC hashing power to compete at the current difficulty. The most challenging task here is to find the right balance between first and second POW difficulty, and how to adjust this autonomously in a way compatible with the current difficulty adjustment scheme.