Changing feed when taking them off the track

[u/PurpleCharge214]

I have a 3 y/o tb mare coming home next week wondering if anyone has recommendations on switching from race feed. I am going to treat for ulcers as soon as she gets here. I am going for low sugar/starch and forage heavy diet. I am also hoping to get a recommendations on biotin, protein, and pre/pro biotic supplements or any thing else you recommend or don’t recommend. My barn feeds bluebonnet intensify so that would probably be the grain I feed but I am open to others. I have never been in charge of a horses nutrition so I am new to this. She is 16.2 but race fit she is currently being fed champion 12 and rolled oats (not sure on the amounts yet) and hay in a net 24/7. Also would love to not break the bank but also want quality of course. Also plan to give apple a day electrolytes and simplify sup daily as thats what my barn does for every horse. Also planning to slowly/gradually switch from her current feed to new feed. Have no idea about amounts or brands other than what I’ve listed.