In the old editor, when I clicked "Link" then "Edit" on text or an image with an existing link, it would bring up the "Insert Link" box. In this box, all the text in the "Link URL" field was highlighted, allowing me to immediately paste a new link into the space. With the new editor, the cursor still defaults to the "Link URL" box, but it is not highlighted, so there's an extra step to highlight the text in order to replace it. I find myself missing this step often, because I am so used to the field text already being highlighted.
This issue was brought up once before, but the original poster didn't respond to any clarifying questions and the thread was closed: