system state after freebsd-update and before portugprade -af

[n dhert]

When doing a major version upgrade of FreeBSD, the last (mandatory) step is
to rebuild and reinstall all third party software (ports)
(# portupgrade -af )

I have a system with 750+ ports, I guess the portupgrade -af will take
something like 30 hours to compile
(on a test system with 425 ports portugprade -af took 15 hours).

What happens in the periode that not every port is yet rebuilt and
reinstalled?
Will applications (webserver, print server, X server, and the many other
apps) not be working,
until these ports (and the ports they depend on) will be successfully
recompiled ?