The 38mm straps fit 40mm watches. The Apple Watch Series 4 comes in new sizes (40mm and 44mm instead of 38mm and 42mm), the good news for Apple users is that the old watch bands fit the new sizes. The 38mm and 40mm watches use the same (smaller) strap range. 42 mm and 44 mm watches use the same (larger) straps.
Apple Watch Series 4, Series 5, and Series 6 come in 40mm and 44mm case sizes, so they're the same. As long as you get the same size for the new Apple Watch, the existing bands should fit seamlessly. In case you didn't know, they're only compatible with Apple Watch SE, Series 4, and newer models. This is because when Apple slightly increased the size of the Apple Watch case, they kept the same strap sizes.
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. Until Apple makes a big design change with the Apple Watch, any band you have that works with the square shape will work. The watch comes in a new case size of 41 mm and 45 mm, moving away from the 40 mm and 44 mm size that has been the standard of the last generations of Apple Watch. Getting a larger Apple Watch band basically ensures that it will work with previous wearables that were the larger size option of their respective generation.
We've combined the new 41mm Solo Loop in English Lavender with the 40mm Series 6 in aluminum (blue), as well as the new Nike Sport Loop 45mm in black with the Apple Watch SE 44mm in black aluminum. 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. Apple Watch 38mm models can be equipped with 22mm wide straps, and 42mm Apple Watch models can be equipped with 24mm wide straps. Weinbach's information that the new Apple Watch “will use different straps that won't be compatible with old watches is open to interpretation, at best.
If you have an older Apple Watch Series 0, 1, 2 or 3, your straps should be for the 38mm or 42mm case. Apple Watches have two screen sizes, 42mm and 44mm, but both models have the same mounting slots for the watch 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. And the specifications indicate that the 44 mm (height) watch is also 4 mm wider than the 40 mm (height) watch.
Apple Watch Series 6 has an always-on display that's brighter than before, so you don't need to make that awkward wrist movement to see the time.