darwin_8 variants on Darwin 9 / Mac OS X 10.5 Leopard

So, we have people who had MacPorts installed in Tiger, on which MacPorts selected the darwin_8 variant. And then they upgraded to Leopard and retained their MacPorts installation. Now they've got Leopard, but their installed ports still say darwin_8. Further, when they go to upgrade these ports at some later point, MacPorts still tries to use the darwin_8 variant. Surely this is bad. What are users expected to do when upgrading the OS major version? Are they expected to throw out their MacPorts install and start over? That would be inconvenient. I'm recommending people uninstall all ports that have darwin_8 variants and reinstall them. That's inconvenient too. What can we do to improve this situation?