No, I think you'll need to do a clean install. That's not such a big
deal (unless you have a slow internet connection for downloading the
.iso) if you have a separate Home partition.
One clear reason for that is all your apps will need to be upgraded as
well. However if you follow the tip I posted last week to create a list
of apps, and then submit it to apt-get after the install, most of your
apps can be reinstated, and from the 64 bit repositories. At least
that's my understanding of it all :-)
Until fairly recently the 64 bit versions were considered less reliable
and advanced and supported than the 32-bit. There's still some truth in
that although it seems to be a lot less of an issue now. There's quite a
lot of helpful discussion in the various forums, and ways round most
problems. But you do need to do a bit of research and experimentation,
and have some fallback.
However the processor capacity isn't all that important, it only really
comes into play if you have more than 4GB of RAM, which is the upper
limit of a 32-bit processor. Even then, the 64-bit width introduces some
inefficiencies of its own (on which I'm a bit vague: but I've read that
it does) so you won't necessarily notice much difference. Video or audio
editing are most likely to benefit, but even then it's more about using
great chunks of RAM than anything else.
What I'm doing is running my test machine on 64-bit 10.04, while keeping
the day to day box on 9.10 32-bit. I've also got a dual-boot on the test
machine with 9.10 32-bit sharing the Home partition, just in case Lucid
can't get to what I need on there. I haven't needed that so far.
Ironically the main machine has 8gB, the test only 4, so I'm intending
to move the main box onto Lucid 64 when it's officially released -
providing there are no show stoppers on the test machine.