Basically there are two ways. First one is sudo emerge -e @world but you don't want to recompile everything do you?
This is were our second way comes in, we will use package.accept_keywords to to unmask our currently installed packages and with enough time our system gets more and more "out-of-date" until we fall back to stable.
First we need to get the list of installed packages and tell portage to not downgrade them: (make sure to run everthing as root)
equery list '*/*' | grep / | sed 's/^/=/' > /etc/portage.accept_keywords/temp Also make sure to not forget to edit the /etc/portage/make.conf and change ACCEPT_KEYWORDS="~amd64" to ACCEPT_KEYWORDS="amd64"
This should be it! Run this to check for upgrades:
emerge --sync && emerge --update --ask @world