I just spoke to a Universal rep on their customer service line (800-864-8377, press 0) and asked when the WB previews were taking place tomorrow. She said they are happening tomorrow and Thursday (though I'm not sure if the rest of the info she gave applies only to tomorrow, since that was the day I first asked about, or to both days). She said the times are 9am-1pm and 1pm-5pm, depending on which slot you are assigned to. WB guests will be "mixed in with park guests" in Wizarding World, but will get a wristband to identify them as part of WB. I'm not sure if the wristband grants priority or exclusive access to any of the attractions, meaning others would have a harder time getting in. She did say that WB guests are likely to face "a bit of a crowd" due to Wizarding World being open to others, so it seems pretty likely that non-WB guests are allowed in, assuming her info was correct (she did have to check a couple times with another person to answer my questions).
I'm hoping to go tomorrow and I can't decide if this is good (open to public) or bad (big crowds and wristbands) news!