There is some controversy surrounding this, and I'm not sure about the exact wording in the NY state laws. But typically, a pre-ban lower will make the entire rifle legal assuming the lower was built into a rifle before 1994. Many people have requested documentation from the manufacturer to prove that their lower was pre-ban, and assembled. The gray area surrounds stripped pre-ban lowers. No one is sure if a stripped pre-ban lower (one that was never assembled into a fire arm) will make a rifle legal. Build at your own risk.
Pre-ban upper doesn't matter. I believe in all states, the lower is considered the firearm (controvery surrounding whether the lower needs to be assembeld or not). This is the only portion that has a serial number, that requires registration, etc.