To keep it simple, I would probably just start adding senior feed now, add it on top of her current feed.
I feed my lactating mares senior, I just add it to their regular maintenance ration; if any are losing weight before foaling, I add it sooner, otherwise I just top up their regular diet with senior feed. I don't even switch, just add it on top of their regular diet.
Most of my mares get about 1/2# of their regular ration daily, to that I've added 1 cup of senior to their evening meal (might have to up that a little bit, but the grazing seems to be doing the most for the lactating mares). They all share hard feed in the morning, where they are getting a mix of alfalfa pellets, timothy pellets and a complete pellet; they'll share this nicely (the senior is molasses coated, and not all need it, so I only use it in their evening meal where they are in separate stalls for dinner).