Seriously, if I thought I could add to the outstanding advice that has been offered here, I would, but I can't.
I believe in keeping your word as well. Honesty is the way you deal with this problem as has been mentioned. Tell the D2 coach the truth (that you still may be open to the D1) and let the chips fall where they may.
After my son's junior year, there was basically one D3 school that recruited him hard. The coach called our house every day and my son and I thought very, very highly of him. He pressured us immensely to commit. Upon visiting the school in the fall, he really made the hard sale. We told him we would commit after hearing from one D1 school and after the D1 signing period. We were completely transparent with them but made it clear we were looking for the D1 opportunity. The D3 coach tried to downplay it by saying all he would get was a recruited walk-on and his odds of playing there would be slim. That kind of turned us off a bit but we still believed in this coach's ability as he led his team to Appleton this past year. The D3 was our backup plan and I don't think that coach would have turned us down if the D1 offer did not materialize.
Honesty and transparency is the solution here imho. Otherwise, keep your word as has been posted.