Hi David,
Just for you're information, I know from personal experiences that a down hill horse could be change buy regular training, lack of training just like human exercises might result in down hill problems, if you take a look at human abdominal specially on mens you will find that if we let things go the billy will come out and the back will arc, same thing will happened with horses back.The best exercises for that is the trot for a period of 15 to 20 minutes head down with a good and fast stride that will push up the back of the horse, you have to strengthened the back muscles, this takes time and patience but the results will come. I bought a QH mare in 2005 she was 4 years hold she had that problem even the owner thought nothing could be done 6 months after she looked gorgeous, she still looks good today.
Regards
Johnny.