Will upgrading to Mavericks break my Macports?
Macports provides a guide to migration to a new OS.

Basically you need to uninstall all the ports, install a new Macports and Xcode (although only the command line tools are needed for most ports), then reinstall the ports. If you don;t do this then you might well have issues which Macports community can't help you with.

If you put your configurations in new files rather than directly edit files installed by macports these files will not be overwritten. If you directly edited the config files then you will need to copy them from a backup after the reinstall.

Note on the upgrade. Xcode does not provide a gcc compiler for OSX 10.9 and Macports is now using the corresponding C++ standard library. This means that any C++ based port could well have problems if you just kept your old Macports and did not go through the migration. I suspect that other code is likely to carry on running but when you upgrade a port or add a new one things will get confused.