One that simply transitions progressively from SHA256 to a single cryptohash, I don't think it would be very complicated. I guess it depends on how the transition is coded concretely.
To keep things simple, there doesn't have to be any transition at all, just a flat 5% payout to the new miners. That percentage would be increased in a new SF only in the event of miner misbehavior. On the other hand, if we do decide to go with a gradual transition, I don't see any great technical complexity with that either. Only it would have to be
very gradual, otherwise we can't expect any support from the existing mining community.
This is how the system might work:
DRAM miner solves the block with no Coinbase TX but with his payout address appended. DRAM miner broadcasts block, solution and payout address to SHA2 miners. SHA2 miner adds DRAM miner's proof to the Coinbase and a 0.625 BTC output to DRAM miner's payout address in the Coinbase TX. SHA2 miner then solves block as usual. Block is now secured by two PoWs.
Some additional protocol (or messages to existing P2P protocol) will be required for DRAM miners to relay their data to the SHA2 miners, but other than this they don't need to coordinate in any way.
The only additional work required for verifying nodes is to check that payout address and amount are correct and verify DRAM miner's proof.
Not quite sure how we'd handle the matter of difficulty retargeting for the new PoW. This seems to be the trickiest problem.
Would be nice to get some feedback on all this from one of the devs.