If you’re buying a band designed for the Apple Watch Ultra, this message is included:

…the 45mm bands work with 42mm, 44mm and 49mm cases.

That means Apple has maintained band compatibility from the original 42mm Apple Watch all the way to the Series 8 version of the new 49mm Apple Watch Ultra.

If you’ve bought smaller  bands for 38/40/41mm

Bands for 38mm, 40mm, and 41mm cases are compatible with cases of all three of those sizes. Bands for 42mm, 44mm, and 45mm cases are compatible with cases of all three of those sizes.