Is it safe to upgrade all outdated port when upgrading from Snow Leopard to Lion?
macports.org currently says:
"Note for Lion users: There is no official release supporting Mac OS X 10.7 yet, but you can test the MacPorts 2.0.0 release candidate if you like."

I usually don't upgrade to a new major version of OS X until MacPorts has released their first official support for it.

I agree with Jerry Jacobs that you should probably back up your /opt hierarchy before attempting. Then you could try doing sudo port selfupdate to see if that upgrades you from 1.9.2 (or whatever) to the 2.0.0 release candidate.  If it does, then you can try sudo upgrade outdated.

I wouldn't try doing a sudo upgrade outdated on Lion without at least being on the 2.0.0 release candidate of MacPorts.