This is easy to do, happened to me a lot of time after reinstall or migrations, but it hard to find information on the internet about how to do this simple procedure.
You should not delete the new keychain.
You just have to open the old keychain and copy the itens on it to the new keychain
- Open KeyChain Utility
- Go to menu File -> Add Keychain (and locate the old keychain on the open dialog)
- Your old keychain will be included on the list of keychains, just below your 'login' keychain [you need to know the password of it, to be able to add it]
Once added, you will see your passwords on it, and can copy/paste the itens you want to your new keychain.
You can choose to always keep the old keychain showing on the application, or you can remove it, and keep it as a backup, or delete if everything was copied to the new keychain, it is up to you.
- To remove it, select it, goto File -> Delete keychain.
It will ask if you want to delete the real keychain 'file', or just 'Delete References`
Don't select "Delete References & Files" !
Select Delete References, and it will be remove from the keychain listing. Only removed. The keychain file will be on your disk, intact, and you can reopen it anytime if you need.
After adding it, right click on it, select UNLOCK.
Then select all itens of it, and Drag and Drop to the new Keychain, they will be copied (without asking for the password of each one)
Third way: (at least exists on Catalina)
After adding it, right click and unlock it, right click again and select "Make Keychain Default"