How to recover change addresses from your old wallet.dat backups

I put together this quick guide that can help folks get back funds that got lost in change addresses from their old wallet.dat backups. You don’t even have to move any funds to check if this method works! You can just create a wallet (let's call it 'wallet A' like in the guide), make a backup, and then create a bunch more addresses, which gives you 'wallet B'. Restore 'wallet A' on a different computer and you should find the private keys that correspond to 'wallet B'. That means you’re all set! If you decide to share this guide, please do it under the CC BY license and use the PDF as it is. If this helps you out, I’d love to hear about it! No donations needed, just let me know it worked for you, that makes me happy. The aim here is to help people, not to make a profit. And a huge shoutout to bias for pointing me in the right direction on how and where to share this. Thanks a ton to an amazing member of this community!

If change addresses are not automatically recovered from a backup, that seems like a serious bug/shortcoming in Bitcoin Core to me. A couple of questions. You write that you need the public keys, but the instructions never use them. When you wrote "public key", did you mean "address"? How do you find the public keys (or perhaps addresses) generated by the wallet in an explorer?

It seems you're a bit confused. What do you mean by this: "Works for Bitcoin-like UTXO QT wallets only"? Why UTXO is included? that's unspent transactions. If you're talking about a wallet, then it must be Bitcoin QT or Bitcoin Core. I think you'd better change or correct the guide from your PDF file. And the thing is, why would you import a  public key just to recover the change address? The change addresses should be recorded on wallet.dat and can be able to recovery by dumping all keys from your wallet.dat using pywallet. If you have an unencrypted wallet.dat file by using Notepad, you should be able to find those change addresses under the pre-generated keys reference code below, and next to that it will show you the private key and addresses. If you were talking about a corrupted wallet.dat, I think the only solution to recover the keys from that wallet is to salvage the wallet.

Public Key = Wallet Address for this context. The instructions do indeed use them in point #4 The REASON you should have this on hand is so you can be clear how much $$ you have in those. To find your change address, you can open the explorer of your coin and track the change addresses used when you sent money from your original wallet address (which you know because it's present in Wallet 'A'). You're speaking of a different scenario. If you have Wallet B and you're able to dump keys or recover them you don't need to use this method. Please review the scenario I've mentioned in the PDF, it emphasizes that you have addresses for which the private key was not backed up and is not recoverable. Or the latest wallet.dat is lost (disk failure or suchlike)

This guide makes no sense. There is no need to call importaddress for every change address; all of the change addresses are already, or will be, in the wallet after keypoolrefill. All that will probably happen is that you confuse users because any of the change that is detected via the import will be shown as watchonly first, until the private key is generated. If you have a backup of your wallet, all you have to do is perform a rescan. Hell, most of the time you don't even need to explicitly rescan because the wallet will rescan by itself when it sees it is not up to the chain tip. You literally just have to load it and wait.

You're talking about a different case where you have a backup of the latest wallet. Which doesn't match the scenario I've stated. I did come across this issue for an altcoin and had to do this in order to have the wallet retain valid addresses. Rescan (starting from an old wallet) and Salvage (on the corrupted wallet in my case) did not yield results. However importing the addresses in advance before keypoolrefill resulted in the right outcome and I did recover all of it. But if it doesn't make sense to you feel free to not use it! Thanks for taking the time to check it out

No, I'm talking about a backup of a HD wallet, made at any time. I have, in fact, either written or reviewed all of the Bitcoin Core wallet code. This guide feels very much "do a bunch of magic and it magically works", without understanding why any of it actually works. I would expect that, if rescan did not work, a keypoolrefill followed by a rescan would be enough. Importing anything should be completely unnecessary. I also want to point out that importing your change addresses will result in breaking change detection.

Fair enough, and I can confirm I've not encountered this scenario on Bitcoin Core or Litecoin or any of the more popular (and thus better maintained?) crypto wallets. One specific usecase where keypoolrefill and rescan did not result in an outcome was with a lesser known altcoin named 'Shibacoin'. When I had a wallet break it basically 'lost' all funds that were on change addresses. Repeated keypoolrefills and rescans didn't do much better. However importing the change address where the funds were sitting (as checked from the explorer) brought results. I didn't completely understand your point about 'change detection' breaking by using this method. Can you elaborate?

Determining whether an address is change is done by some data being missing from the address book. Explicitly importing a change address will set that data, which means that the address will no longer be considered change. This results in some transactions being shown in a confusing manner where change is shown as a new incoming output rather than elided as change.