Apple Support response regarding 38mm bands compatibility with the newer 42mm Series 10 case size.

[weeknd]

I'm currently using a 38mm Link Bracelet with the 41mm Series 7.

I got so many different answers regarding the compatibility of a 38mm band on the newer 42mm Series 10.

I think I'll take my 38mm Link Bracelet to the Apple Store once the new watches are available in-store and ask the staff there if I could try out my old band on the newer 42mm Series 10 to confirm it or buy the new 42mm Link Bracelet along with the 42mm Series 10.

 

[G3Blue]

Please see my post here, this shows 38mm is not compatible with 42mm.
I guess it will fit but likely not a perfect fit.

https://forums.macrumors.com/threads/series-10-42mm-bands-apple-states-bands-for-42mm-44mm-and-45mm-cases-are-compatible-with-each-other.2437588/

 

[Pug72]

My wife's 38mm 2015 Modern Buckle straps fit seamlessly on her 42mm S10.

 

[ssledoux]

I feel like they are doing a poor job of making that clear.

The 38, 40, 41mm bands all fit the new 42mm watch.