When upgrading within a major version number (6.x to 6.x; 7.x to 7.x; 8.x to 8.x, etc) you shouldn't need to upgrade any ports. Check /usr/ports/UPDATING and /usr/src/UPDATING to make sure.
When upgrading across major version numbers (6.x to 7.x, 7.x to 8.x, etc), you will have to recompile all your installed ports. You can also install the sysutils/compat* port that corresponds to the old version of FreeBSD, which will allow you to continue to run your installed ports without recompiling ... but the long-term solution is to recompile.