They would need to do another hard fork. They don't need anyone to vote for it, much like last time ( only 20% voted ), they will just fork. This would essentially create ETC ( Original Blockchain ) - ETH ( PoW BlockChain - ETH-PoS ( PoS Blockchain ). All three will still exist and you would be able to mine on two of them. People might not want to adopt ETH-PoS and the market price might not be glorious.
Here is more information about it:
-Epic