Phil, let's unpack this issue a bit. Seeral things at play: the bit count of the OS, the bit count of the scanning application, what the scanner driver does and what the scanner software does. I'm using Windows, so I'm less familiar with how this would work on a Mac O/S, so let us stick with Windows for the moment. I am given to understand - haven't tested it myself - that if I run a 64-bit OS I need 64 bit apps to work with it. To operate a scanner I need two things: a scanner driver which works with the OS on my computer, and a scanning application which directs the driver. The two need not be in the same bundle and often aren't.
Now, for all those folks, like me, who have high quality film scanners (for example a Minolta DSE 5400 1 or 2, a Nikon Coolscan in one of its various vintages - all discontinued hardware but top of its class) - the drivers for these scanners are TWAIN and designed for 32 bit OS. regardless if the driver, users have the choice of using the manufacturer's bundled software for adjusting the image parameters before scanning, or using a third-party application such as SilverFast or Vuescan for the same purpose, or making no adjustments and doing everything post-scan in Lightroom or Photoshop.
For people who own such scanners (and there are very few options left on the market for replacing them at comparable image quality) contemplating an ugrade from 32-bit to 64-bit OS, or switching from say Windows 32 bit to Mac 64 bit, where does that leave them with their scanners? Do they become book-ends, or are there linkages of compatibility that would allow OS, scanner driver and scaning application to play well with eachother? Does one need to maintain the hardware and software on hand to switch back to 32-bit come hell or high-water to use these scanners, and if so, what's the best way to achieve that? Not clear to me that we've parsed this one fully yet. The answer could be it depends, it depends...........