about switching 4.0_BETA2 from i386 to amd64

I've got an amd64 box still running i386 port.

I would like to change architecture.

I've tried
./build.sh -m amd64 -O ../obj -T ../tools -x -U -u tools
./build.sh -m amd64 -O ../obj -T ../tools -x -U -u distribution
./build.sh -m amd64 -O ../obj -T ../tools -U -u kernel=GENERIC

but i can't boot with the new amd64 kernel (and everything else i386).
also

./build.sh -m amd64 -O ../obj -T ../tools -U install=/

complaints about trying to install on /

ERROR: 'install=/' must != / for cross builds

how must i do ?

greetings
luca
-- 
http://www.sironi.tk                   fax 02-70035867