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Ethereum's main network is getting closer to the new Altair network, which means that the absolute majority of the nodes on the network have to update their clients. According to the Nodewatch service, currently around 30% of nodes remain unsynced.
The new upgrade for Ethereum is aimed purely at validators, and it does not change anything for general coin users. The update affects only the beacon chain consensus mechanism. In addition to users, updates will not have any effect on contracts.
If users act as validators, they will need to download the latest version of their software, which they will be able to find in the table on the Ethereum blog, and restart their nodes as usual.
If node holders decide not to update to the latest version of the software and join the upgraded chain, they will be synced with the old chain after an upgrade on Ethereum occurs. While functioning on the incompatible chain, users will not be able to receive proper rewards and realize them on exchanges.
Validators running on the old network will receive penalties for not participating in the upgraded consensus with respect to the upgraded mainnet.
When the Ethereum community has decided which changes should be made to the network, the change is written by the clients functioning on the network. The changes to the beacon chain protocol go live in a specific epoch (the Altair update goes live at epoch 74240).
Any new nodes with the old ruleset will remain abandoned on the old chain without being able to use new functions, participate in the current network or, in general, utilize the cryptocurrency.