Can I downgrade Apple Watch SE 2 from watchOS 10 to watchOS 9?

This seems to be an option, since the Apple Watch SE 2nd Gen. had originally watchOS 9 installed.

Have a look at this thread:

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Hi,

My problem has been solve.

I finally insisted to Apple support. Apple support sent my iWatch to the factory for a downgrade.

In less than 7 days, I have my watch back with the appropriate software to be paired with the iPhone 8.

So, I highly recommend to insist with the Apple support. They found the solution without having to buy a new phone. Thanks Apple

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copied from: Why won't my Apple Watch SE pair with my … - Apple Community

Sue if you want to buy a SE2 you can and then use it. You cannot however downgrade the version of WatchOS.

If you want to buy a new watch and you only have iphone X or 8 then we have seen reports here that Apple have downgraded to earlier WatchOS but you need to have that agreed at time of purchase.