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I've tried to understand what's happening under the hood when encrypting a wallet. Valuable resources has been #8389 and #8383.
Encrypting the wallet will
- Create a new HD seed
- Save a new HD seed and the old HD seed to an encrypted wallet.dat file on disk.
If the above is correct, then the GUI is misleading (because the underlying process has changed since this GUI was created?). The process in v0.16.0 is as follows, including my suggested improvements:
Select File --> Encrypt wallet... This is misleading because it gives the impresstion that it's just encrypting your existing seed. I suggest the menu item text "Create encrypted wallet..."
Dialog

The window title is misleading here too. It should read "Create encrypted wallet". The text in the content area should mention that this will create a new seed and that the old seed will be kept too, but is not used to generate new keys. Maybe a checkbox "Save old seed too" could be useful here (default: checked)?
- Dialog

The title should be "Confirm create encrypted wallet". The questions should be replaced by eg "Are you sure you want to create a new encrypted wallet? Your old keys will [keepSeed?"still":"not"] be available in your new wallet"
- Dialog

Window title should possibly read "New wallet encrypted", but existing text is probably ok. The IMPORTANT warning is very confusing. The old unencrypted backups will be perfectly usable, but you can't access funds held by keys created since encryption. The warning makes it sound like they are somehow destroyed.
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