It would have seemed strange to go back to calling hu "Professor Lastname" after years of calling hu by hu's first name. But I'm not sure if I did it correctly.
I always struggle with this a bit myself. This informality in the US is sometimes quite confusing.
For me, seniors would always be addressed in a formal way as a sign of respect for their age.
However, I feel that in the US this would almost be considered subservient if this was done when the senior member referred to you by your first name, so I generally try to stick to using the same way they address me regardless of age (well, unless I am meeting older, non-colleagues, then it's generally Mr./Mrs/etc) ... I guess my moniker is well chosen :-|