All 42mm watches that are series 10 or higher you’ll need to wear 38/40/41 mm straps if wearing bands from prior generations in addition to the new 42mm ones.
This size chart from bandbreite explains it. Link here.
I had already been aware that since the series 10 increase in Apple Watch sizes, some people upgrading from series 3 or earlier watches that used to have the old 42mm size would need to be cautious about this size difference.
If you’re transitioning from an Apple Watch 1st generation to an Apple Watch Series 3 and had the 42mm size of these models, you’ll need to get a 44, 45, 46, or 49mm watch from these series 4-9 and ultra models to continue using your existing bands.
A 42mm series 10 or an Apple Watch series 11 of that size would work well with smaller straps that are 38, 40, or 41 mm from series 0-9 prior, and the 42mm bands released in fall 2024 (series 10 release) thereafter. It’s just slightly more rounded and longer than the 40/41mm watches. A newer 42mm band released from fall 2024 onwards can be forwards compatible with a series 9 and earlier 41, 40, and 38mm watches.