The new 49mm Apple Watch Ultra will work with most existing Apple Watch bands, according to Apple's website. The device is compatible with bands designed for the larger 42, 44, and 45mm case sizes available for the Series 8 and older Apple Watch models.
It is not, however, compatible with some of the smaller bands that are designed for the 38, 40, and 41mm Apple Watch models. From the Apple Watch site:
You can match most bands with any Apple Watch Series 3 or newer case of the same size. The 41mm bands work with 38mm and 40mm cases; the 45mm bands work with 42mm, 44mm and 49mm cases.
The Solo Loop and Braided Solo Loop bands are only compatible with Apple Watch SE and Apple Watch Series 4 or newer. The 40mm and 41mm cases work with band sizes 1-9; the 44mm and 45mm cases works with band sizes 1-12.
Some bands, such as sport bands purchased for the 41mm Apple Watch, won't work with the 49mm Apple Watch Ultra, but bands designed for the larger Apple Watch models are compatible. Basically if you have a band you've used for the 42, 44, or 45mm Apple Watch models, it should work with the Apple Watch Ultra. Apple also suggests that all Braided and Solo Loop sizes will work with the new larger Apple Watch.
Conversely, the Trail Loop, Alpine Loop, and Ocean Band crafted for the Apple Watch Ultra can also be used with 44mm and 45mm Apple Watches, but not the smaller 40mm and 41mm models.
