Landing on the correct lead shows you are preparing in advance for the new direction, most certainly you will not be marked down for it. If you can do a figure 8 WITH a fence the horse will understand more clearly as well.
So, how to introduce changes: if the horse is on the forehand they will likey not change united (usually late behind). So getting the horse active and positioning in the working canter is necessary (balanced). Use half halts to prepare the change, change flexion/change direction, asked. But a trainer can help you by a. Riding the horse to prepare it, and the b telling you what the timing is.