In this very specific case, where you want to change the anchor text for a correct link, you can make this change, but it doesn’t have anything to do with ziplinks. To change the anchor text “foo” to “strong” you move the cursor to the blue link text “foo” and edit it like normal text to say “strong.” This kind of editing of anchor text is possible for all Tinderbox links.
On a practical level, when I’m doing this, I usually start somewhere in the middle of the blue text, type what I want the new link anchor to be and then delete the extra text before and after what I’ve typed because I find it easier to manage. That’s not necessary though. You can do it however makes sense for you.