Consider the following statements:
All unicorns are pink.
Some unicorns are pink.
The former is true, since there are no unicorns that are not pink. The latter is false, since there is no unicorn that is pink.
The classical conception of logic apparently operated on the assumption that we'd only ever logically quantify over meaningful subjects--that is, that we'd never have a vacuously true statement like the first one above. For more detail about the relationships between quantified statements in classical (Aristotelean) logic, look at this article on the so-called "square of opposition" (in particular, up through the "Modern Squares of Opposition" section).
In your case, you could drop the "all" down to "some", but only if you knew that you were quantifying over a non-empty collection of individuals. For example, we couldn't do this if we were talking about leprechauns. However, if we were talking about guys named Vito, and we also had the statement "Some guy is named Vito," then we could drop the "all" down to "some" as described. In other words, the following would be a valid argument:
All guys named Vito are Italian.
Some guy is named Vito.
Therefore, some guy is Italian.