Shouldn't we keep the original copyright?
If we remove all references to the original implementation (which we are reusing), that looks problematic to me (thinking of being accused of plagiarism here).
Point 4 c) above seems to point in this direction.

I am not clear either. The license states that "You may add Your own copyright statement to Your modifications and..." so should it be as follows?

Copyright 2016-2022 All in Bits, Inc
Copyright 2023 Informal Systems

Agree, probably a good idea to consult with legal what is the right way to do this. Looks like you can add your own copyright (but probably keeping the original). As per item 4:

 You may add Your own copyright statement to Your modifications and
      may provide additional or different license terms and conditions
      for use, reproduction, or distribution of Your modifications, or
      for any such Derivative Works as a whole, provided Your use,
      reproduction, and distribution of the Work otherwise complies with
      the conditions stated in this License.

We had a conversation about this last year when replacing the Tendermint Core license with the standard Apache 2.0 license (see tendermint/tendermint#9293).

This particular part of the license is the appendix - it has no legal bearing on the license. It makes no sense to apply a copyright statement within the appendix. See https://opensource.stackexchange.com/questions/4149/include-appendix-of-apache-2-0-in-license-file/4151#4151

If copyright is applied, it has to be applied elsewhere in the codebase afaict.

From a grep through the codebase at tag v0.6.6, there are no copyright notices in the tm-db codebase apart from this modified entry in the appendix (which can be omitted from the license altogether, since the license is defined as sections 1 through 9 up to the END OF TERMS AND CONDITIONS statement - see clause 1).

To be on the safe side, could we "move" the original Copyright here somewhere, .e.g. in the README. So that we can't be accused of having "removed" their copyright?
Then we'd add ours just below

But then we'd just be applying a copyright for AiB in a place where they never applied copyright. So we'd be creating a copyright restriction on their behalf. Not sure if that's legal, since we don't have the authority to do that.

Eventually we will need to modify the content of CONTRIBUTING.md in the cometbft repo to reflect these changes, they still mention tendermint

I am not clear either. The license states that "You may add Your own copyright statement to Your modifications and..." so should it be as follows?

Copyright 2016-2022 All in Bits, Inc
Copyright 2023 Informal Systems