The Ethereum network will be undergoing a scheduled upgrade at block number 9,069,000, which is predicted to occur on Saturday, December 7, 2019. The exact date is subject to change due to variable block times and timezones.

Check https://etherscan.com/block/countdown/9069000 for a countdown to the event.

What should I do as an imToken user?

Just like any other Ethereum update, Istanbul is supported by imToken. As a user you neither have to worry nor do anything.

But why an update? What changes for me?

The Istanbul network update is technically speaking a scheduled hardfork of the Ethereum blockchain. As such, it will include a couple of updates that previously went through the governance process in form of Ethereum Improvement Proposals (short: EIP).

Check https://eips.ethereum.org/EIPS/eip-1679 to find a list of updates, or read the articles linked below for more details. In short, the changes that go into the update:

EIP-152: Add Blake2 compression function ‘F’ precompile. Helps with interoperability between Ethereum and Zcash as well as other Equihash-based PoW coins

EIP-1108: Reduce alt_bn128 precompile gas costs. Makes certain privacy solutions and scaling solutions cheaper, including Matter labs, Aztec Protocol, ZK Rollup and Zether

EIP-1344: Add ChainID opcode, allowing contracts used in Layer 2 protocols such as Plasma to track the correct chain, especially during hard forking

EIP-1884: Repricing for trie-size-dependent opcodes. Prices are always optimized for a trade-off between DDOS resistance and smart contract usage costs

EIP-2028: Calldata gas cost reduction. Helps with zk-STARKs applications.

EIP-2200: Rebalance net-metered SSTORE gas cost with consideration of SLOAD gas cost change

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Ethereum Istanbul Upgrade Announcement