Since she's getting into more mature years, definitely monitor how well she can chew hay. Older horses tend to need things soaked or even in place of things that younger horses require.
You want to have no sugar in her diet if you can help it. LOTS of fibre, protein, fat, and vitamins and minerals are important.
I recommend talking with your vet about putting her on a good value senior feed.
I loved having Sky on Triple Crown Senior with loose salt and hay 24/7. He now is also on Cosequin (concentrate) and MSM for his joint health. I check his teeth twice a year since he tends to have hooks appear every 8-9 months.
It may also be a good idea to think about running a blood panel on your horse to check for any deficiencies after you begin the new diet. www.FeedXL.com
is a fabulous tool. It costs $50 per year or less for smaller increments of time.
Now what exactly do you do with her? That could gauge how much grain she requires for energy.