Ethereum Beacon Chain Upgrade: Altair Beacon Chain nearing completion, what are the impacts?

According to the announcement by the Ethereum Foundation, the Altair beacon chain upgrade is expected to go live at epoch 74240, which is scheduled for October 27, 2021, at 6:56:23 PM.

What do Ethereum users and Ether holders need to do?

According to the announcement, this upgrade is purely a consensus mechanism upgrade for the beacon chain and will not affect the current PoW Ethereum chain where most users are. Therefore, end users, smart contracts, etc., will not be affected.

Only operators running beacon nodes on PoS need to upgrade their nodes.

What do operators running beacon nodes need to do?

Download the latest version of the software and follow the instructions to restart.

What happens if you don't upgrade?

Nodes that do not upgrade will cause a fork and will remain on an old, incompatible chain.