As the tin says, how early could Japan have begun Westernizing, and what might it have taken for such to occur?
Might be worth walking through a "Oda Nobunaga survives Honnō-Ji and wins the unification wars" to see where it goes ... he was a genius in several fields (military and economical) and was both the first in Japan to mass field firearms, and reorganized the military so it was talent instead of family that decided who's who in the pecking order, while the economy was changed (or at least attempted) from agriculture based to manifacture/service based, and moving towards a pseudo-free market model, with monopolies prohibited, much to the disagreement with the guilds.
Tokogawa 'merely' dusted off some of his (easier to understand) plans when they won the shogunate.