IOM i would say that doing just a simple loops the judge will not be looking whether you on correct and should have not effect on mark, as judges should be looking a a good rythrm, balence etc... and if by changing diagonal through movement it effect ryrthm, you'll drop marks. If you normally change at home when training horses, and it doesn't effect rythrm, balence etc... go for it. Getting correct leg is important when your asking for an exention within pace ie working trot - meduim trot
But i'm not a judge but there are a few judges and they can defo help -
If you mean loops off the outside track to the 3/4 line then no, I don't think you do as you aren't changing the rein and your bend is only changed briefly. If you are crossing or joining the centre line and then changing the rein, then yes, you should change diagonals on the centre line.
Hope this makes sense as I am not sure exactly what you mean and it's been a while since I did a dressage test! I'm sure someone more knowledgable than me will correct me if I am wrong.
This makes me remember a discussion I had with a judge over this and pretty sure this was the outcome.
I did a PC test and changed my diagonals and on my sheet was told off for it, now I thought this was incorrect as to help the horse stay balanced its important to stay on the correct diagonal. When I was on ponies I was lucky to get a lot of assistance from some very good list 1 judges (mainly as organisers in Jersey were very good at bringing in some very good judges!) and I think it was Sygrid Young in this case I was speaking to (sadly now passed away but very respected judge) and she said you should change diagonal for the section on opposite bend unless you are sure you can maintain the rhythm and balance without changing, which is very rare from a prelim rider or you are doing sitting trot which again is quite rare in a prelim rider to the extent they don't 'dampen' the trot. She said a good judge would never mark down for a change of diagonal unless your riding was that poor it caused the horse to do something completly odd
Infact a few judges may actually pick up on the fact that you didn't change!
Ok after all that rambling yes you should change your diagonal for the opposite bend section of the loop (eg second half) in order to maintain the balance and rhythm unless your the likes of Carl Hester in disguise
I do it for BD, and my trainer tells me to do it too - never been marked down for doing it. Dont know what the rules are for BE mind....
As you reach the widest point of your circle you obviously change the bend and change diagonal, just before you hit the track you change bend change diagonal again - if your horse is well balanced this should be a fluent movement.
To be honest, I was a bit thrown when I saw the print out of the test showing the loop. It is effectively drawn as a half circle coming off the long side with the widest point being the centre line (ie 10m) - not quite how I picture a loop in my mind when I ride it.
I was riding them a couple of nights ago and it felt ok to change the diagonal. Will try it without a change tonight and see which has the least effect on the trot and stick to that as there doesn't seem to be a cast iron answer.
Pretty sure there isn't a 'rule' for it but, as I always change diagonal when schooling normally, changing diagonal on the loop before I change the bend helps set my horse up to change direction.
I was told that for Eventing dressage you wouldn't be marked down for what we in the UK consider to be the wrong diagonal, because in other European countries they sit on the opposite diagonal to us or don't bother with diagonals at all ! Don't know how true that is??
Agreed DC... why would you want to change diagonal... you are staying on the same rein!! but if you changed and unbalanced the movement, then it might raise the odd eyebrow!
Sorry I have not properly read all the replies as I am watching xtra factor at same time.
But I personally think that you shouldn`t change the diagonal as you are not changing direction, but just slightly changing the bend and to change the diagonal twice, would probably unbalance your horse, and the changes would be too quick in succession to look smooth.