We have our decisions made. A 20% chance is not good enough for us to blow the entire savings (we have some money saved up for this). We would need higher chances than 20% - that's a little low. 40% or higher I would go for it. That being said, we do all we can to prevent colic. You can't always prevent it, but there is a lot you can do to keep your horses' gut healthy, and an ounce of prevention is worth a pound of cure in my book. So far, we've been fortunate in that regard ... we've had other horse issues, however - they are horses, after all.
When I am worried about a change of diet, I load up on pre/probiotics for my horses more than their usual dose (prevention) and, if it looks lush, I use grazing muzzles if they are going to be out, and I try to limit their time. If limiting time is a problem, at least try to get the horse on some pre/probiotics (I like Animed Remission or Manna Pro Optizyme, but a lot of people use Probios) and keep on a fleece/sheepskin padded, breakaway grazing muzzle. They can eat and drink with the muzzle, they just eat a lot less.