This is an interesting question because it speaks to how various movements relate to each other. Whatever figure or movement is being performed, the rider’s first job is to ensure that the basic principles of dressage are honored. This is the Training Pyramid stuff and the qualities enumerated in the Collective Marks. Beyond this, each movement has a set of criteria which should be fulfilled – these are listed in the first part of the Dressage section of the rulebook with fairly elaborate descriptions of each one.
So here’s the deal: If you ride a half pass where a leg-yielding is called for, it’s not a better leg- yield. You get punished, not rewarded because you haven’t fulfilled the criteria, namely the direction of the positioning or bend that is called for. The same thing happens if you make a simple change instead of a change of lead through the trot.
If the test calls for a 20 m circle, and you make a 10 m circle you are also not rewarded even though it’s more difficult. It’s just assumed that either you don’t know your figures or you can’t steer.
On the other hand, if a Third Level Test calls for a turn on the haunches and you make a walk pirouette instead, you haven’t made a different movement. By the definitions and by convention you have just made a better turn on the haunches (as long as all the criteria are still followed). In this case the score will be higher. The same would be true if in Fourth Level you perform an actual canter pirouette when only a Working Pirouette is required. The rulebook is quite specific about this.
In a similar way an extension is just a better lengthening. It still fulfills all the requirements that we need to see in a lengthening. It just has more volume and expression.
If you are doing Training Level on a big mover, you can dial 10% of a lengthening into your Working Trot, essentially saying to the judge, “This IS my Working Trot.” You may not be able to get away with that in First Level if you can’t make a lengthening that looks different enough from the working gait you have chosen.
[As an aside, in Second, Third, and even Fourth Levels, most typical riders cannot show a qualitative difference between their Mediums and Extendeds. Technically a medium should have more lift and a higher carriage than an extended. By FEI, this is more believable. Tactically speaking, in the medium levels if you don’t have a very good extended, it’s not too smart to show a lesser version of it for your medium. Then you get a mediocre score in both blocks. It’s often wiser show the biggest one you can in both, even if they are identical, and let the judge figure out how to score them. That way you may get a decent score for your medium and then one point lower for the extended. ]
Taking the larger thread one step further, if you show too much collection – like if you make passage when Working Trot is called for or if you do a whole test in Very Collected Canter when only normal collection is asked for, you’ll be docked for not complying with the criteria as it relates to ground cover and over tracking. As you can see, these can be fairly complicated issues. Fortunately, they are topics we discuss at judges’ forums and trainers’ conferences. They aren’t supposed to be secrets (like the Dressage Handshake) that you aren’t allowed to be let in on.