How to convert from i386 to amd64

[george+freebsd at m5p.com]

The following are probably all frequently-asked questions, but I haven't
been able to find the answers yet.

1. How can I compile an amd64 kernel while running an i386 kernel?  The
CPU I'm on is and amd64, if that matters.

2. Will i386 applications run on an amd64 kernel, or do I have to update
the whole world before I can do anything after installing an amd64
kernel on this machine?

Thanks for your assistance.                     -- George Mitchell