I got my FR 235 yesterday and when I got home I immediately connected it to garmin express. It asked me if I want to replace my old device(FR 225) with the new one for activity tracking and I answered yes. After doing that, my steps data yesterday which I have given a lot of effort to achieve is gone.
I'm sure there was a warning in there where when I select "yes" my data will be deleted but this logic is wrong. It would mean that for you not to lose data, you need to transition only at 12am.
Transitioning to a new device should add the data from the old one to the new one.