Canon Terms of Address
My dearest flist, whom I have neglected horribly, I need your help again. If Lupin were to visit Hogwarts as a guest (to avoid spoilery-type terms) post-PoA, how would Dumbledore address him, if he were speaking in front of the school?

Problem is, I know he'd normally address his (past and current) professors (or most people), by their first names, but what if he's being formal?

The scene in question, horribly cliched, but it's a draft:

All conversation ground to a halt when they entered, then resumed in not-so-discreet whispers. Dumbledore stood as they approached the High Table, his ancient face lined with sorrow.

"Mr. Lupin, welcome." For once there was no twinkle in the old man's eye. Remus shook his hand, knowing the entire hall could hear their exchange, for it had fallen silent once more.

"Thank you, Professor Dumbledore."

Opinions? I can't recall a scene in canon that would give me a clue, and I don't even know where to begin looking.