The bands for 38mm, 40mm and 41mm housings work with each other, and the bands for 42mm, 44mm and 45mm housings work with each other. Most bands designed for Apple Watch Series 4, Apple Watch Series 5, Apple Watch SE, Apple Watch Series 6 and Apple Watch Series 7 work with any previous version of Apple Watch. It's unclear if the reason bands like Milanese Loop aren't available is because Apple simply doesn't want to ship any more straps that say 40mm or 44mm or because the Milanese Loop will require a redesign to match the flat-edge case. Thanks to its larger screen, Apple Watch Series 7 will have a full keyboard with QuickPath, a fully optimized user interface and new watch faces.
The new Apple Watch will be available in five new aluminum colors, three new stainless steel colors and two titanium finishes. That means that a strap on a Series 3 watch could be noticeably too small for Series 7, since a 3 mm difference between 42 mm and 45 mm could be a bridge too far. I think it's much more likely that there are some new bands designed for Series 7 where backward compatibility only goes as far as Series 4, such as Solo Loop and Braided Solo Loop. Although it wasn't the redesign that many thought it would be, the Apple Watch Series 7 looks slightly different from last year's model.
When you buy an Apple Watch band, you may need to specify which watch it is for, large or small, but you can usually get any of the bands for any of the watches. If the strap is properly installed, it will not slide freely until you hold down the strap release button. Apple's own watch bands are currently in short supply, with the classic Milanese Loop strap and many seasonal bands currently not available for purchase at Apple retail and online stores. Apple Watch Series 7 will be announced soon, almost certainly on Tuesday, September 14 (here's why).
Since the older Series 3 bands fit all Series 6 and SE, Apple Watch fans have plenty of straps. I find it absolutely incredible that Apple risks driving these fans away by saying, upgrade to Series 7 and its collection of watch bands will be reduced to a single usable strap, the one you get with the new Watch. So, that means Apple has to make new straps that are compatible with at least the SE or make straps that will only work for this year's models, once again unlikely to the point of being unthinkable. Solo Loop and Braided Solo Loop bands are only compatible with Apple Watch SE and Apple Watch Series 4 or newer.
Older bands that fit the 40mm Apple Watch Series 6 will fit Series 7, and the 44mm straps will fit the new 45mm size.
