As for the future, and how to prepare the tree to heal smoothly, you'll want to clean up the bark left behind the callus (orange lines) by carefully scraping it away with a dull blade. Kathy had me use the trowel end of my tweezers. A knife would work, just scape with an "away" motion so you're not applying force with the cutting edge.
Then, using the sharp edge, open the callus on the rolling edge (green lines) to keep it moving. Done now, it may close by the end of the year. Then, you'll need to clean up the seam that results.
Here is a visual on my zelkova.