The Bitcoin chain will be defined here to be the Proof-of-Work chain (of the ones which are direct descendant of the Bitcoin chain in 2023) which has the most hash power (using the SHA256 hash scheme Bitcoin in 2023 uses) supporting it.

A block reward mechanism change is any change which affects the way miners are paid. Currently, miners are paid by block rewards, which halve every 4 years, and transaction fees, which come from transactions on the network. Any of the following would count as a change:

• Changing the rate at which the block reward halves, or keeping the overall rate the same but changing the payouts to not be constant over the four-year period.

• Any EIP-1559 like mechanism where the fees from transactions are no longer given directly to miners.

The following would not count as a change:

• Changes to the cryptography or script code governing transactions, as long as it's still possible for transactors to include fees for miners.

• Changes to the default algorithm for which block to build on (i.e. mining on random blocks at a given height as the Selfish mining paper suggests, rather than the first block), as long as the basic permission structure of a miner being able to build on any block they want is still in place.