Find a place to go, get a stall and just let it see the bustle and…important…hear the PA without having to go race. No matter how calm at home, they often get worked up when returning to that competition environment. They think they are going to run and maybe some bad memories of rough handling come back triggered by the sound of the PA and activity in the barn area.
The other thing that trips even the best prepared up is working as part of a large group in a very small area. Not a skill developed at the track. Gets even green horses that never raced upset, cannot be duplicated at home. Although it does help not to ride alone all the time, nothing prepares it for the chaos of the schooling ring.
Those two things cause the more dramatic reactions in ex racers. The more you can expose them to that before actually going in a class, the easier you will make it for you and the horse. Just haul and hang out if you can’t get a stall or it’s a one day. You just need to erase the race part out of their memories when going off the property to a competition where they will be part of a large group.
Not particularly advocating for it but, since you won’t be actually showing, if it gets upset or overwhelmed, bit of Ace might be appropriate. Best to try without but with so many other horses around, some handled by novices and kids, it’s a real bad place to manage a meltdown.
That said, many of them are fine, no issues at all. Just take it slow and be conscious of the varying experience or lack thereof of others at the show. Which is another reason to plan on not showing the first time or three off the property, you need to be sure of what you have before actually going in that small show ir schooling ring with a large group.