To be clear, the attachment mechanism for the bands is universal between all Apple watches released thus far (and likely far into the future), so yes, it will fit - it will attach securely and connect and disconnect easily. But the watches and bands are available in two basic sizes, effectively (though not called) "small" (38 or 40mm), and "large" (42 or 44mm). Bands from either will fit the other, but will not look right. A "small" band in a "large" watch will look a little too narrow, while a "large" band in a "small" watch will overhang the sides and just look wrong. I'm sure there are people who do it, but I wouldn't recommend it.