I met two people at a past conference and we had a nice conversation there, but no further contact. Recently, I've seen their names on the list of accepted papers of an upcoming conference I'm attending. Since I'll stay at the conference location for an additional day afterwards and don't know anyone else there, I planned to write them an email and ask whether they had any plans for that day.
However, during looking for email addresses, I noticed that they are both full professors, which I wasn't aware of before. I'm a PhD student. Would it be a breach of etiquette to contact them? While conferences are often used for networking, I don't want to make anyone uncomfortable.