The straps for 38 mm, 40 mm and 41 mm cases work together, and the bands for 42 mm, 44 mm and 45 mm cases work together. Most straps 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. The Apple Watch Series 4, Series 5 and Series 6 come in case sizes of 40 mm and 44 mm, so they're the same. As long as you get the same size for the new Apple Watch, the existing straps you have will fit well with no problem.
Apple Watches have two screen sizes, 42 mm and 44 mm, but both models have the same mounting slots for the watch straps. Apple Watches have interchangeable straps, which means you can change them and change them to different styles at any time. However, just as all previous straps fit the Apple Watch Series 7, these 41mm and 45mm straps are compatible with previous watches. So that means that Apple has to make new straps that are compatible with older versions at least with the SE or make straps that only work for this year's models, once again unlikely to the point of being unthinkable.
Apple claims that this new watch is brighter indoors and is the most durable Apple Watch in history, with IP6X dust resistance, WR50 water resistant and a “crack-resistant” display. Leaks and conflicting rumors prior to the presentation suggested that, therefore, the new 41 mm and 45 mm Apple Watches would require new straps. While the Apple Watch has been around for many years, Apple has maintained the same tab design every year. Before, there were only a few strap options available on each Apple Watch model, but now you can choose any combination you want.
I think it's much more likely that there are some new bands designed for Series 7 where backward compatibility only reaches Series 4, such as Solo Loop and Braided Solo Loop. If you're the same size as the new Apple Watch, your existing straps should fit right in. In addition to maintaining the same mounting slots as the watch strap, smaller 38mm and 40mm screens also have smaller screen sizes. We saw reports that the edges of the Apple Watch case would be flat, similar to those of the current crop of iPhones.
Apple completely redesigned the Apple Watch Series 7 screen to create more than 50% more screen area than Series 3 and 20% more than Series 6.Remember that only four versions of the Apple Watch have the current dimensions: Series 4, Series 5, Series 6 and SE. Until Apple makes a big design change with the Apple Watch, any strap you have that works with a square shape will work.