Hay is going to be the same O&A mixture, and she will slowly be allowed more access to grass. Trying to keep everything the same for her for the most part. Trying to find more info on feeds for horses prone to tying up has mostly just been the Selenium, Vitamin E and carb levels and percentages.
She is on supplements now for Vitamin E and Selenium, as well as an electrolyte though.
I'm just so lost! Gone is the time when I just accepted whatever my trainers fed my horses in the past!
You are in Seminole Country! I used to feed Seminole feeds for years - and very happily. I had a (suspected) EPSM horse 10 years ago and they were helpful. I was on first name basis with their DVM Nutritionist - Larry Mack. Don't know if he is still there or not. Their Wellness line (I think that's the name) is a lower starch/higher fat product. They had a distribution problem for a while in my area and I ended up converting over to Triple Crown. It would be worth you checking Seminole out since they are local to you.
The Perform Safe (while also being a good all-around higher fat pelleted feed), is specially designed for horses that tie up. IMHO, this would be your best bet. All of my guys are on one type of Wellness feed or another. One mare gets the Compete Safe, the sweet feed equivalent of the Perform Safe. (but the Compete Safe is not especially targeted for horses prone to tying up)
The Show and Sport is their highest fat feed in the Wellness line at 12%, but does not have the yummy herbs that the other Wellness sweet feeds do. (the Calm & Cool, Compete Safe and Senior) Personally, if my horse had a chronic issue with tying up, I would stick with the Perform Safe. If she needs something with a higher fat content to help keep weight on, I would look into maybe adding beet pulp, rice bran or Buckeye's Ultimate Finish. Seminole and Buckeye have a distribution agreement, so you should be able to get the Ultimate Finish through your Seminole dealer. Good luck!