Steps to reproduce:
- Open a page that already contains an external link without a label: [http://example.com].
- Try to change that link to [http://example.com anything]. No fair removing the whole link and re-adding it.
Yeah, I couldn't figure out how to do that either.
(According to the bug titles, bug 51309, bug 53505, and maybe some others address this, but the descriptions don't seem to match the titles very closely.)