It won't be like SegWit/Unlimited. The politics are very different and the technology is very different.
1. Politics. Ethereum has a leadership team which functions very differently from Bitcoin Core. For one, the creator of Ethereum is well known and outspoken. He and the other members of the Ethereum Foundation made it clear that their intention was always to move to Proof of Stake. Though there are some who may not support this move, the general community bought into the project knowing that this was the plan from the beginning. And there's a lot of trust of Vitalik.
2. Technology. Ethereum adapts much more quickly to mining difficulty than Bitcoin. Bitcoin can't survive with 90% of the hash power on one chain and 10% of the hash power on another chain. The smaller chain would take 10 hours to mine a single block and months to readjust the difficulty. It would be unusable. Ethereum has already shown its resiliency to a split during the DAO hard fork. The smaller chain adjusted quickly and survived. What this might mean is that with the change to PoS, there could be a dissenting faction that remains on PoW, and both chains will survive. This will allow the PoS chain to go ahead and push through with a split easily even without 100% support.
Ethereum has a difficulty bomb (mining becomes more difficult with time). This is a form of control from developers over blockchain. If miners refuse to cooperate, they are left with the blockchain which is more difficult to mine with time (and eventually unusable).
1. Politics. Ethereum has a leadership team which functions very differently from Bitcoin Core. For one, the creator of Ethereum is well known and outspoken. He and the other members of the Ethereum Foundation made it clear that their intention was always to move to Proof of Stake. Though there are some who may not support this move, the general community bought into the project knowing that this was the plan from the beginning. And there's a lot of trust of Vitalik.
2. Technology. Ethereum adapts much more quickly to mining difficulty than Bitcoin. Bitcoin can't survive with 90% of the hash power on one chain and 10% of the hash power on another chain. The smaller chain would take 10 hours to mine a single block and months to readjust the difficulty. It would be unusable. Ethereum has already shown its resiliency to a split during the DAO hard fork. The smaller chain adjusted quickly and survived. What this might mean is that with the change to PoS, there could be a dissenting faction that remains on PoW, and both chains will survive. This will allow the PoS chain to go ahead and push through with a split easily even without 100% support.