When only changing a config option on an installed port -for example adding a feature you missed initially- it is sometimes a hassle with all the ports installed as parts of the one to be reinstalled, especially if it is a metaport like php5-extensions or the like.
To not have to also update all the ports this reinstall will crash on I like to do the following:
make deinstall && make -E FORCE_PKG_REGISTER=1 reinstall distclean
This will simply ignore all those pesky “already installed” errors.
And since I am not actually upgrading anything but rather reinstalling the same thing again this will not brake anyting.