https://wccftech.com/uh-oh-your-current-apple-watch-bands-wont-be-compatible-with-series-7/

It seems as though Apple Watch Series 7 bands won’t be compatible with any other Apple Watch models.

You can use 38mm Apple Watch bands with the 40mm model. Bands can be used between 42mm and 44mm models as well. However, it seems as though this flexibility is about to change with the release of Apple Watch Series 7.