I work as manager in Apple authorised service centre UK, and this is directly copied from GSX article on servicing the Apple watch, does not mention other bands.
AC+ Coverage
1) AppleCare+ (AC+) covers Apple Watch and all in-box accessories.
That is all... The people you all talk to on the phone and also the "genius" guys are only people who are knowledgable in Apple products, they do not service them, they haven't a clue most of the time with technical questions ��
This might of course explain why the model number, visible in Settings, General, About says a specific model that has a bracelet combination. e.g. mine whows as a MJ3X2B/A which reveals it as a 42mm SS with Black Classic Buckle.
I wondered why they might go as far as making the model number specific to the band on the watch body.