Not really any option to do that currently. 12.04 is in Alpha with
daily builds that will add/remove/break/fix things as the final code
is worked out. Only the 12.04 release candidate will be upgradable to
the full version and that's not expected for a while.

You can try the Alpha builds and then the Beta ones are they are
released but each one would need a clean install and certainly not
recommended for anything other than 'getting the flavour' of 12.04
with the understanding that you'll be needing to start all over when
the final version is released.

I've been playing with the 12.04 Alpha myself and whilst it's pretty
good for an Alpha version it's still got a lot of raw edges, mainly
around the installer itself. Try if by all means, but be aware it's
got a long way to go yet :-)

K> IF I were to want to start early, what would be the logical version
K> to install that would be upgradeable to 12.04?