I noticed, once I set the target page for a Nav Link, I no longer am able to change the target page to another page or remove it in the Nav Link setting menu.
There is a workaround to make it work. If I change target from "Another Page" to "External Link" and go back to "Another Page", the target page setting is cleared and I can set the link to a new page.
I am not sure if this is by design or something else. If I change the target page to link up the Nav Link to another page, that change should propagate on all Nav Bars instances used in other pages. On a second thought, this might be out of scope for Nav Bar and we should expect such capability to be supported when Elements (or whatever you will call them) are introduced in Webflow.