"upgrading" NetBSD/i386 to NetBSD/amd64 ?

Hi there, 

I have a Pentium4 EM64T machine currently running NetBSD/i386.  Is it
possible to "upgrade" it to NetBSD/amd64 using the standard build.sh route?
i.e. compiling a NetBSD/amd64 distribution, installing an amd64 kernel and
extracting the amd64 userland over the existing i386 one?  What about
packages?  I will recompile them all, but will the currently installed i386
packages run on the amd64 base system?  

What CFLAGS (-march=?) are supported?  

Thanks for any advice, and please CC me.  

        Geert