play around with half halts and going for medium/extended in each pace
To lengthen you would use more leg pressure but half halt and use your core so that they're don't go into the upward transition. To shorten you half half and use very little rein. It's hard to explain but once you feel it you'll understand.
To lengthen you have to use more leg to get them forward but more half halts so they don't run away