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.  If miners behave badly, users will turn their backs to them, and it will become more difficult/expensive for the miners to create new blocks.  Another important advantage is that old nodes and wallets will still work, as long as the majority of the hashrate is behind the soft fork.  Still safeguards must be put in place to stop someone with a very large SHA256 hashrate to overtake the main chain at a later time when the extra POW difficulty is high, since a SHA256 only chain will still be valid for old nodes and SPV nodes.