Downgrade
Four months ago my aunt bought a series 8 watch 45mm. Can we downgrade the 45mm series watch to 41mm or even SE series?
Apple Watch Series 8
Question marked as
You'd probably be able to trade it in but that wouldn't be for full price paid.
You'd want to see what sort of trade in value you could get and compare that to selling it yourself.
If you sell it to an individual, please make sure that you make it ready for them by completing these steps:
Sell, give away, or protect a lost Apple Watch - Apple Support
Also, the Series 8 is available in a 41mm case, but the smaller case for the SE is 40mm.