As far as I can tell, once you've downloaded the 64-bit app, you can launch it instead of the older version and it's an exact drop-in replacement. For example,

As far as I can tell, once you've downloaded the 64-bit app, you can launch it instead of the older version and it's an exact drop-in replacement. For example, all my open tabs were restored in the 64-bit process, as they had been in the 32-bit one.