Converting FBSD installation to DFBSD?

[walt]

I have a machine with a little-used FBSD-4.9 installation which I
wouldn't mind sacrificing to try out DFBSD.
So, I was wondering if I could just cvsup new DFBSD sources to
replace the FBSD sources and proceed to compile the new system
that way.
Will a FBSD kernel run executables which are compiled for BFBSD,
I suppose is the obvious question.  Will the installworld
die in the middle when some critical bit of userland gets
suddenly replaced?