Can I upgrade an economy ticket to business with points later?

Rarely. Most 'Saver' award tickets (the ones you can book with partner points) are non-upgradable. You should book the business class seat directly using points.

Is it better to book round-trip or two one-ways?

Always book two one-way tickets. It gives you flexibility to change your return date or fly different airlines on the way there and back.

Booking Business Class to Japan with Points

To secure a business class seat to Japan with points, you must book exactly 330 to 355 days in advance when award seats are released. Focus your points on transfer partners like Air Canada Aeroplan or Virgin Atlantic to access Star Alliance and SkyTeam inventory efficiently.

Identify your transfer partners. Don't transfer points to airline programs immediately. First, look for the 'sweet spots' like transferring Chase/Amex points to Virgin Atlantic (for ANA flights) or Aeroplan (for United/ANA flights). Each program has different rules for Japan routes.
Use search tools to find availability. Use United.com (for Star Alliance) or AmericanAirlines.com (for Oneworld) to search for 'Saver' level business class awards. If you see it there, it is usually bookable through other partner programs.
Book exactly at the schedule release window. Airlines release business class award seats 330–355 days out. Set a calendar alert for 350 days before your intended return flight. If you don't book at midnight on the release date, these seats will be gone.
Transfer points and confirm. Once you verify the flight exists on the partner site, transfer your points from your credit card portal (Chase, Amex, Capital One) to the airline program. Transfers are usually instant, but they are irreversible. Confirm the booking immediately after the transfer.

Can I upgrade an economy ticket to business with points later?: Rarely. Most 'Saver' award tickets (the ones you can book with partner points) are non-upgradable. You should book the business class seat directly using points.
Is it better to book round-trip or two one-ways?: Always book two one-way tickets. It gives you flexibility to change your return date or fly different airlines on the way there and back.