Say it out loud once, then let the relationship stay the same everywhere except the 1:1.
Name it in the first real conversation. Something like: “I know this is different, I was your peer last month and now I'm the one signing off your time off. I'm not going to pretend that's not a little weird.” Naming it costs you nothing and buys you a lot of trust, because everyone on the team already knows it's weird. Pretending otherwise just makes you look like you're avoiding it.
Then watch for the two ways this actually goes wrong. The first is overcorrecting into Boss Mode: suddenly stiff, suddenly formal, running meetings like you've got something to prove. People notice that faster than you'd think, and it reads as insecurity, not authority. The second is undercorrecting, keeping the exact same joking, no-filter relationship you had before, which works fine until the first time you need to give real feedback and neither of you knows how to have that conversation anymore.
The fix for both is the same: keep the relationship, change the context. You can still grab lunch, still have the same sense of humor, still be the person they used to vent to about the old boss. What changes is that in a 1:1, you're now asking about their goals and giving them real feedback, not just catching up. Draw that line early so it isn't a surprise later.
One more thing nobody tells you: they're watching how you treat them to figure out how you'll treat everyone else. Go easy on them because you used to be equals, and the rest of the team notices immediately, it costs you credibility with all of them, not just the one person. Hold them to the same bar as everyone else. It's actually the kindest thing you can do, it tells them you still see them as capable, not as someone who needs to be handled.
This answer helped?