It could be fair to force them recall the watches if they remove the blood oxygen feature, people might have bought the watch with a set of functions, and removing them should invalidate
This is an import ban, it should have no effect on already imported watches.
And, in addition, there is a limited exception to the import ban for the purpose of replacing/repairing existing devices. People who bought the watches before tomorrow (or whenever imported supplies run out) are not affected.
Some of the news coverage I saw a few weeks ago said that this is true for in-warranty repairs, but not out-of-warranty repairs. Obviously this wouldn't apply to any newly-purchased devices, which have a 1-year warranty. But it could affect owners of Series 8 watches, for example, if they haven't bought AppleCare (which extends the warranty past 1 year.
Yep, only in-warranty repairs (including AppleCare). If your device is out of warranty and the "repair" actually means a full replacement then you aren't getting a replacement with blood oxygen enabled.