Research indicates that reducing protein intake can have a greater effect on milk production than decreasing calories.  Therefore, while it is possible that you may have a small effect on milk production by cutting out the sweet feed and replacing it with balancer pellets, I don't think it will be notable because by switching to a balancer pellet with >25% protein you are not actually giving less protein, just fewer calories. The greater stimulus for milk production is milk consumption by the foal.  The more milk the foal drinks, the more milk the mare will produce (within normal physiological limits).  If your change in diet does not affect the milk intake by the foal, then you would not see a drop in milk production.