I'm a first time homeowner with very little experience caring for trees. Apparently the guy who owned the house before us knew even less than I do and has pruned three trees quite poorly, leaving stumps 6-12 inches long. It looks terrible and from what I understand is not an appropriate way to prune.

Is there any problem with re-pruning these stumps to cut them at the collar? I know this will at least look better, but will this help the health of the trees at all?

The trees are a black walnut, an ash, and some type of maple with much smaller leaves than a silver maple. Thanks.