The Ethereum Protocol Upgrade Hard Fork is to occur in September 2017. What will happen to Ethereum's price?

The upcoming Metropolis hard fork is expected under consensus in Ethereum community.

This means that we are not to expect any serious negative price swings of ETH because there is no fear or uncertainty regarding this fork.

See the following Ethereum Hard Forks Timeline:

As you can see, with the exception of the DAO in 2016, Ethereum has gone through a number of hard forks with no split in the blockchain.

There is a number of Ethereum Improvement Proposals (EIPs) that will take effect under Metropolis hard fork and all will greatly improve Ethereum.

The upgrades in code will make for a cleaner, leaner protocol which in turn will ensure smoother execution of contracts on the chain.

The protocol upgrade (Metropolis) will therefore enhance the perceived value of ETH. However, this enhancement is already factored in by the market.

If we are to expect anything following Metropolis, it should be a medium positive effect on the price, but nothing too spectacular. At least not because of this fork.

Of course, with any hard fork there is always the chance that some nodes will just continue with old version, effectively creating an alternate Ethereum.

If you are holding ETH right now, even in the event of split you have nothing to lose: as it happened with Bitcoin and BitcoinCash hard fork, you will be holding BOTH your ETH and the ALT-ETH that will come out of the fork.

Chances of seeing this happen with Metropolis is very very small.

Chances of split may increase with the more controversial upcoming Serenity upgrade. Changing from Proof of Work to Proof of Stake may effectively kill investment in mining gear for Ethereum. But that is another story - we will be following closely!