Why would you want to do that? Even so... Basically all it takes is to empty (/ remove) /usr/ports. After that you can use portsnap(8) to grab a new copy. Another option is to simply runHi how do i remove the entire ports tree so i can re download the entire ports image?
# portsnap fetch extract, this will fetch (download) and install (extract) the new copy.
Apart from what SirDice said also keep /usr/ports/CHANGES as well as /usr/ports/UPDATING in mind. Sometimes things can change, and if ports were removed then re-installing the ports collection won't bring them back.Just that some src ports have gone missing, and a
portsnap fetchand a
portsnap updatedon't get it back.