So I had a Header Download-Watch List in which I'd chosen the wrong Look in Groups. When I realized that, I thought I'd try this...
Edit the Watch List, choose the correct Look in Groups, right-click Watch List, Re-scan Topic/30 days. Seemed logical but I really didn't know what it was supposed to do.
Loading the Watch List, what I got was far too many posts. Most of which did not match the Filter Profile I had selected for the Watch List.
Easy enough to fix, I just selected the Filter Profile in the results.
My imagination of what Re-scan topic would do: Scan the current Look in Groups as though those headers were being downloaded, apply the Filter Profile, and add the matches to the Watch List. Does that sound right?