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.
> What about if you had pages open that you don't want to lose (literally hundreds of tabs)?
There's always one of these in every discussion about a web browser. That one person who, instead of bookmarking things, keeps literally hundreds of tabs open.
Then you should first change your browsing habbits, and probably several other ways in which you organize things in general. And ask yourself if you really understand modern technology.
Second, you could save all the open tabs as bookmarks, with the Bookmarks -> Bookmark all Tabs command. It will save all open tabs in a new bookmarks folder.
Lastly, when you reopen the browser after the update, you can click File -> Reopen closed tabs, which will reopen all of those, even if you haven't bookmarked them.