Have any (cryptocurrency + blockchain)s switched from proof-of-stake to proof-of-work?
Don't know any cases.
The thing is that if you start the blockchain on PoS mode, it doesn't have much sense to change to PoW because:
- The Tx/s rate will be much lower.
- It would force validators to invest money in components with much higher computing capacity than they would probably have at the time. And at the same time, it would increase their expenses in terms of electricity.
- It will open the door to attacks that are difficult to carry out in PoS but not in a blockchain in which the miners would still be few and with few resources. We talk about attacks like the 51% atack.
Think about that, a 51% attack is so much harder to succeed in a mature PoS blockchain than on a recently addapted-to PoW blockchain.
There is one implementation that mixes the two Proofs (i know its not passing fron one to other) but maybe it's useful to you. I'm talking about HCash.
Here i leave you one article that talks about the benefits of mixing PoW + PoS:
Hope it helps.