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?
Since it's an upgrade, it means positive for the holders.
You'll be holding a new coin and I believe the price will rise because it has been expected that this upgrade in development will solve
the problem in the past. So, if you are here for the tech and to make profit, better start buying now, price will likely to skyrocket soon.