The Ethereum hard fork called Constantinople is expected to occur between January 16th and 17th, 2019, more precisely at the 7,080,000th block, and it is a fork that will lead to the transition from Proof of Work (PoW) to Proof of Stake (PoS).
In addition to reducing transaction costs and mining rewards, this fork also improves the execution of smart contracts and paves the way for the transition from Proof of Work to Proof of Stake.
To date, Ethereum, like Bitcoin, uses PoW as an algorithm to validate blocks, but this makes system scalability more difficult, slower and more expensive.
In other words, PoW is seen as a limit that could negatively affect future developments of Ethereum.
So, while for Bitcoin there is no change on the horizon, on Ethereum instead the transition from PoW to PoS has long been discussed, which would make the network much faster and cheaper.
However, in order to switch to PoS deep changes to the protocol are necessary, which require several hard forks. One of these is Constantinople.
Therefore, after the introduction of the five above changes, the new Ethereum protocol will no longer be fully compatible with the old one, and it will be necessary to update the software in use on Ethereum in order to continue to use it.
The five changes introduced with the 16th January hard fork will mitigate the inevitable impact that the transition from Proof of Work to Proof of Stake will cause in the future.
So, after the 7,080,000th block, PoS will not have been introduced yet, but thanks to Constantinople the Ethereum network will be more ready for its introduction: to date, in fact, it is not yet clear when PoS will actually replace PoW on Ethereum.
Looking at the Ethereum 2.0 roadmap, the Constantinople hard fork can be considered a smaller fork, useful to lay several foundations for the replacement of PoW with PoS.
In particular, the short-term objective is to increase network capacity and reduce mining energy consumption, while the medium-long term objective is to lay the foundations for the definitive transition to Proof of Stake.
However, during 2019 both Proof of Stake and Plasma could arrive on Ethereum, and perhaps Sharding in 2020.
In a couple of years, Ethereum could be a very different protocol from what it has been until now.