I clicked on the title of this COTH thread thinking I was going to read some gossipy chat about high stakes dressage classes and underhanded betting schemes.
Instead, it's a discussion of whether or not it's okay to randomly start riding whatever figures you feel like in the middle of your dressage test, if you feel that makes the ride better. Say, you feel like a 10m circle would really help out your right bend, so if it's okay to just throw one in. The OP frames the question as schooling show with a tense horse, so that helps maybe a little bit, but I am utterly horrified.
No. No, it's not okay to start riding whatever you feel like in the middle of a dressage test, even if you're okay with getting dinged for error of course. That is not the point of a dressage test, which is a proscribed series of movements in specific places and specific times, one leading to the other. The transitions are part of the test.
Not only is it missing the point, it's incredibly, staggeringly rude to the judge, to the show organizers, and to your other competitors. It's wasteful of the time and effort everyone has put in to craft a schedule, to choose tests, and to put on the schooling show in the first place. I have scribed for many, many dressage tests and if I ever saw someone randomly dropping in additional movements just because they felt their horse needed it, I would be confused and pissed off, as would every judge I've ever worked with.
Now: are there situations in which you can essentially school in a show ring? OF COURSE. Here's the difference: you have a conversation with the show organizers ahead of time. You say, we're having a lot of trouble with the show atmosphere; would it be possible for me to ride my test just before a break, or during a break, or at the end of the day, or first thing in the day? You make it clear that if things go drastically wrong, you might need to stay in for a few extra minutes to school and fix it. You are polite and courteous and you don't just do whatever you feel like.
I've done that. I've asked to go back in the ring at the end of the day. I've completely, utterly blown a test and, knowing there was a gap after me in the schedule, asked if I could go back in the ring and work it out for a few minutes. Most schooling shows are totally fine with that; it's what they really want to do. In fact, I've scribed for judges that have sent me as a runner after a horse and rider that just exited the ring to ask them to come back and work in the ring for a bit after they've had serious problems.
But just deciding to ride something different on the spur of a moment? No. That does not make you a considerate rider who's just doing the best for her horse. It makes you a jackass.