Here's what I have to do now:
1. View the bookmark I want to edit in the Inspector;
2. Type (or paste) the corrected URL into the "Suggested URL" field;
3. Click the blue "Show URL, Last Verify Result" icon in the bottom-centre of the Inspector window to hide the opened panel;
4. Click the same icon again to re-open the panel, showing my edited URL in the "Suggested URL" field;
5. Click the button between the "Current URL" and "Suggested URL" to switch them.

Hiding and re-opening the panel is non-intuitive, extra baggage. Failing to do so before switching URLs will keep the original value for "Suggested URL," regardless of any changes. This is hideously bad usability.

Either:
a) Handle the appropriate NSText notification so that your app has the URL value typed or pasted in by the user, or
b) Add another button to the left of the "Suggested URL" edit area which will update the internal value from the NSText control.

I would strongly prefer option a.