ngo' means not new, while qan means not young. One is used for objects and the other is used for living things. You wouldn't call a young man a new man and you wouldn't call a new book a young book.
In Turkish, which also distinguishes between eski (not new) and yaşlı (not young), eski shares with English "old" the meaning "former" -- as in "our old cook [= the 20-year-old whom we had to fire] was no use so now we have a new one".
Though I suppose in Klingon that would have to be vutwI'ma' Deq rather than vutwI'ma' ngo'.