According to my vets, equine senior and equine junior are very similar base formulas, then they are tweaked accordingly (I know there is more to it, but for simplicity sake, there is a basic formula with similar ingredients among particular formulas under the same brand).
With that said, the formula should be pretty similar, but you may need to add the extra oomph needed by supplementing the weanling with calf manna (love giving this to those that are "just lacking" in some way) or something similar. I have, when I was breeding, fed equine senior to all my horses! Including weanlings, with good results, very easy to to digest.