So, Opera does it automatically in this case if you download the default net installer. The upgrade to 64-bit won't move Opera from 'C:\Program Files (x86)\Opera' to 'C:\Program Files\Opera'.
When upgrading a 32-bit Opera, Opera will upgrade it in-place. When you use it for an upgrade, it'll still download the 64-bit version to upgrade with, even if you're upgrading a 32-bit version of Opera. When you launch it on 64-bit windows, it should fetch the 64-bit version of Opera and install it. The download on should download a simple OperaSetup.exe file.