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:

The following would not count as a change: