The last pope to retire without pressure to do so was Pope St. Celestine V
. As he is reported to have spent his retirement hidden away by his successor in a palazzo in Rome, no one needed to know how to address him.
If a pope chose to retire now, I expect that the Vatican would have to announce the protocols to be observed for a former pope. At this point, all that we do know is that only one man at a time can be pope and only the reigning pope should be given the full marks of honor due a reigning pontiff.
As a side note, there used to be an analogous situation with former living U.S. presidents. Former living American presidents were never called "Mr. President" or "President LastName." During their lifetimes, they were called "Mister" or addressed by a former non-exclusive title they had held (e.g., President Eisenhower was called "General Eisenhower" after he left office). My guess is that a similar protocol would be adopted for a retired pope.