Is it a breach of protocol to contact people higher in the academic hierarchy for social reasons? 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.
It is great that you are thinking about networking as a PhD student. 

My advice would be escalate the relationship by one step. Going from a "nice" conversation at a previous conference (where no email or contract info was exchanged) to a social activity outside of a conference is a bit like asking someone you had a coffee with once to go on a trip with you. It escalates the relationship very quickly and could get awkward. Furthermore, many people don't stay extra days at conference locations so it is likely to be unsuccessful.

Less intense ways to connect:


Send them an email saying that you noticed they were presenting at the conference, and you are looking forward to seeing their work and chatting again
Run into them at the conference (as Ian_Fin suggested) and ask what they are doing for dinner or lunch on a conference day