Many of the horses are retired due to some type of hoof problem (navicular disease is a common problem) and this needs to be managed in retirement just as it was prior to retirement. Your horse can't suddenly do well seeing the farrier 2 or 3 times a year when farrier care was needed every 4-6 weeks while working!
To date we have been able to successfully transition all of the retirees to barefoot. Our farrier fits boots with pads for the horses that need them, and this allows them to have a pain-free transition. Don't let anyone tell using hoof boots is easy - they are a pain - but well worth the hassle. What we find is the horses will either have an easy transition and don't need the boots, or they need the boots WITH pads for a period of time. That time period varies for each horse. The boots must be removed and cleaned daily during the transition. The amount of time the horse needs the boots and pads varies with each horse. Some literally only need them for a week or two, others need them for a longer period.
I would never recommend just pulling the shoes and trying barefoot with ANY horse if you suspect the horse will have a painful transition. That is unfair to the horse and the horse should remain shod rather than have a painful transition.
By the way, we have not found the miracle cure for hoof problems. The horses are quite comfortable barefoot on our grass pastures. Were they still in work many of them would need to be shod again, although some might not.
Some pictures of the horses receiving their pedicures: