BumGenius V3 are birth to potty, so they're adjustable to supposedly fit all babies, but you might find that once your LO gets closer to potty training, they outgrow them, not all do, but some do. My DS' still fit fine and he's almost 3.
There's also the possibility that they might just get worn out after 2 or 3 years of use, they last a long time, but not always forever (although I have to say our BGs are the most reliable and well lasting pockets that we have). So if they do wear out and you need to replace them then you won't be able to replace them with more BG V3 as there won't be any more, so you'll have to try something else.
Then of course there's the possibility that by joining CNT you'll become completely fluff-addicted and want to try everything