I changed my Rivals email just before July 1, too, because I noticed my login email was the same for On3, as was my username (from when I used to comment on KSR articles). Around July 1, I slightly changed my On3 username, adding a dash (but have since removed the dash by going to my name in the Menu on the top left, then in the Profile area, though it took a few minutes to take effect). After the migration, I can’t login with my “changed” Rivals email but my On3 account has my Rivals post count as of today, so I am guessing my info was merged between the two, even though I initially changed my email and adjusted the name slightly.
This tweet says our historical posts from the original board will eventually be brought over. You have to click it to see the entire message, but it ends with “…This won’t be ready at launch, but it is part of our integration plan. It will take a little time, but it’s coming.”