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Occasionally developers of a coin will switch blockchains. Examples include ANS --> NEO and EOS ERC20 token --> EOS coin. In these cases, if there is a complete swap of coins and the original coin is abandoned, then there is no taxable event. Instead the new coin takes on the cost basis and holding period of the original coin.
Note: this is not the case in a hard fork where two or more chains continue to co-exist.