I just changed our departure flight day/time for our trip next week at the 3.5 week mark. Price was $10 cheaper.
Then on 7/27, I changed our return flight from 8/14 to 8/15 and the fare was $15 more than previous flight, but I used the $10 credit from above to offset.
In both instances, flights were midweek, not a weekend.
HTH!