The 41mm Apple Watch Series 7 replaces the 40mm Apple Watch Series 6, while the 45mm Apple Watch Series 7 replaces the 44mm Apple Watch Series 6.
Despite the increase in size to 41 and 45mm, the Apple Watch Series 7 is compatible with older Apple Watch bands.New 41mm Apple Watch bands are backwards compatible with the 38mm and 40mm models, so the reverse should be true as well, while the 45mm Apple Watch bands work with the 42mm and 44mm Apple Watch models.
Existing 38mm and 40mm bands should be compatible with the 41mm Apple Watch Series 7, and the 42mm and 44mm Apple Watch bands should be compatible with the 45mm Apple Watch.