According to Wells, the patch was "incredibly easy" to implement because the game already contains preliminary support for Trophies via its Medals reward system. He further revealed that these hooks were actually included because Naughty Dog thought Sony would roll out Trophies before Uncharted's November 2007 launch.
As with other PS3 titles getting retroactive Trophy support, Uncharted will require those who've already completed the game to play through again in order to unlock Trophies, even if they had previously earned every Medal. Wells says this is primarily to prevent people from simply using someone else's save data to artificially bolster their Trophy collection. Shame on you for even thinking about it.