In order to force the activation, you would need to specify which of the two versions you have installed you want to activate, for example:
sudo port -f activate grads @2.0.2_7
But the real question is why does /opt/local/bin/wgrib already exist and not belong to a registered port? That should not be. If you cannot explain why these files already exist, who knows how many other files are in your MacPorts prefix that should not be there. Because of this, you may run into this or other problems again later with other ports. It may be safer to uninstall all ports, and uninstall MacPorts and delete all of /opt/local, after saving any important data therein, and then reinstalling MacPorts and the ports you want. Then you can be sure the only files that are there are the ones MacPorts put there.
