With the increased work, she’d be burning more calories. Calories that have, for the past 3 years, been keeping her at ‘maintenance’. So she’d likely loose fat for a little bit as her body adjusts to the new routine and then start to gain muscle. I wouldn’t suggest a muscle supplement such as SmartMuscle or Tri-Amino. I’ve seen 0 improvement in body condition with my personal experience of both. I’d start with boosting how much hay she gets, give her additional calories to burn off without the hot of additional feed. As for senior feeds, since your mare is active I’d suggest Purina Senior Active Healthy grain. It’s complete feed, has the senior benefits, but also the little extra umph she would need to start getting back to the beautiful Oldenburg you knew 3 years ago. Hope this helped, good luck!