You can probably expect a lighter wallet, and not much else, if the dog doesn't have food allergies. Grain-free kibble is mostly marketing nonsense -- it evokes raw, prey model feeding in the minds of consumers while having nothing whatsoever in common with it.
Grain-free foods are still made with starch. You cannot make kibble without starch -- the extrusion process in the machine requires it! So they just sub out some other cheap starch for the grain: peas, tapioca, or potatoes, instead of rice and oats. That's not always a good substitution though: most dogs with no allergies digest rice and oats very easily, and possibly better
than some of the "grain free" substitutes. So what have you really gained?
I'm all for a rotational diet for adult dogs though --- Fromm is a great company to do that with eventually. My kibble-fed dogs rotate through the Fromm Four Star foods, with grain and without, for the variety of the different protein and veg components in them. I don't think their GF foods are inherently "better" though -- they just offer some nutritional variety AFAIC. That's just marketing hype.
If your pup is doing well on it, I'd feed the regular Fromm large breed puppy food, and put the difference into a new chew toy or pack of bully sticks for your little one!
ETA: I just looked at the ingredients out of curiosity. They're both 26% protein, and the calories are almost identical! The GF doesn't necessarily even have more meat -- it just has chickpeas and lentils high up in the ingredient list. Legumes are not highly digestible for dogs. In the same spot in the cheaper grain-in food, you have oatmeal, barley and rice -- highly digestible.