The attachment is super secure since both the 42mm and the 38mm watches have the same 'connection' size (those 3 black/silver bumps on the bands).Thanks for posting, I was thinking of trying the 38 on my 42 as well. Does it stay in place? My only fear would be if the band wasn't securely attached to the watch, would it come off? I'm really surprised they don't offer both sizes. I know a lot of women to have the 42 and prefer the larger watches. I just went through all the bands, and don't see a single 42 available.
My only qualm is that the 38mm band will be a little smaller around the wrist on a 42mm watch because the bigger watch piece is taking up more space than the 38mm watch would! If you have big wrists I wouldn't recommend using a 38mm band on a 42mm watch.