You do not have to slowly wean off of grain. Simply stop feeding it or give a very small amount if you are not allowed to change feeds. Actually I would not board where they won't feed to my specs. Just in case that is a issue. Regarding IR, just feed her as if she is IR. Whether she tests positive or not it appears she needs to be off the grain. I would go back to what you used before as it worked for you. And that lush grass for 8 hours can do a lot of damage if your horse has any problems with sugars. Is there a dry lot area you can use for part of that time?