The good news is that modifying either into a scrub generally means just moving the frog back to open the mouth and grinding the radius (camber) on the blade. Both of those changes can generally be reversed if the mood strikes later but at the cost of a lot of blade grinding or a new blade. Scrub planes are normally narrower but there is no reason that the Miller’s Falls plane couldn’t be used as a scrub since you can control the width and depth of cut with the blade grind radius and the blade projection. The miller’s Falls also appears to have a wider mouth. Some woodworkers that do a conversion will file open the mouths of their planes but that makes it hard to reconfigure them for smooth-plane use if they change their minds later.
My personal choice would be to spend just a little on one of the Harbor Freight planes and set it up for scrub plane use. While they are not great planes, scrub work doesn’t require a lot of quality or precision to be effective. For about $10 or less you can have a perfectly good scrub. I have a true scrub but, just for the fun of it, have picked up some of the Harbor Freight planes and have set them up for both scrub planes and smoothers. The HF scrub works great and the HF smoother works well as long as the wood has little figure (the mouth is too open). The other option would be to adapt the Miller’s Falls Plane by backing the frog and cambering the blade and try it before changing anything else. One of the things a lot of folks miss is the fact that rough work, scrub or true jack plane (not smoothing ) work can be easily done with a really crappy plane as long as it is set up right for the work. Save the money and preserve the better planes for more precise work. Alternately you could also just grab a cheap old jack of dubious quality and set it up for scrub work. True scrubs are about the same length as a common jack.
Traditional English woodworking didn’t include a “scrub” plane. They just grabbed any convenient plane with a wide mouth and heavy camber that was about the right length (generally a jack) and hogged off the wood. I understand that the German woodworking tradition provided the idea of a plane type specifically designated for scrub work and the plane manufacturers promoted the concept so they could sell yet another plane to workmen. It’s always struck me as funny that the tool manufacturers have “educated” woodworkers on what they “had” to have. Anyone remember when all of the woodworking magazines promoted radial arm saws and even now, there are manufactures that want to convince you that your woodworking square has to be within a thousandth of an inch to be “good” in order to justify an exorbitant price. Fine for those that want them and choose to spend their money that way but most of us would prefer to have an nice adequate tool and spend the difference on another needed tool or lumber. Just remember that all of the wonderful furniture produced during the colonial period that we admire so much was produced with sophisticated, traditional, but relatively simple tools, precise where needed and imprecise where adequate. High end tools will never replace knowledge.