These changes are considered to be against the spirit of Bitcoin. Even if all Bitcoin users decide to adopt any of these changes, the resulting cryptocurrency can no longer be considered "Bitcoin" because it has diverged too much from the original design.
- Increasing the total number of bitcoins beyond 21 million.
- Changing the bitcoin distribution algorithm whatsoever before block 6930000 (the reward drops to zero on this block).
- Recycling lost coins. People use Bitcoin assuming that some bitcoins will inevitably be lost. If coins are artificially "un-lost", the money supply is changed in a way that people would not have expected.
- Any rule that adds required, explicit centralization. For example, a change requiring that all blocks be signed by some central organization.