It’s come to light that the new Apple Watch Ultra has the same Apple Watch band connector as the existing 44mm and 45mm watches.
All three of the large Apple Watches share the same bands. The smaller 42mm watch won’t work with the new bands, and 42mm bands won’t work on the Ultra.
Notably, Apple’s fine print says, “The Trail Loop, Alpine Loop, and Ocean Band are designed for Apple Watch Ultra and pair best with the 49mm case size. The bands are also compatible with 44mm and 45mm cases.”
Therefore I’d suggest that most people wait and see what the bands look like on standard Apple Watches before buying the new larger straps.