What puzzles me is that you'd want to change. With the description that you give, it sounds like you've got at least a decent strikeout/waste pitch. Why change it? If you can get guys swinging on it, that's great, keep doing it. I wouldn't try to throw it slower, because more than likely with your age that will lead to some change of pace in your delivery pace/and/or arm speed. I'd just keep doing what you're doing. If you want another pitch to 'throw for a strike' work on a cutter or something.
Brice