It is not basically an update. It is an upgrade where the whole blockchain is changed, the code is changed, everything you can see in the ethereum is changed.
How will it affect the current ETH holders?
It will really affect the ethereum holders simply because the price might spike again and aside from hard fork they can gain another token will be give after the hard fork. There few days before the ethereum update and I really hope that because of this update it might triggered the price to increase again.