If she’s clean she’s good to go for dressage. Personal preference how you present her feathers, mane/plaiting wise. I don’t leave a saddle patch for summer clips.
But the equestrian centre has 6-12 stay away shows a year to recoup that. The camp site 365 nights (they hope!). It’s often expensive but I see it as a luxury for when times are good or it’s very cold! Most our shows are greenfield so even higher costs.
If you pay for an individual load it doesn’t need to be a multi day shunting effort. Assuming the older horse is in good health I wouldn’t think twice about it.