This change has a number of implications, particularly for those who are executing SQL directly. PDO's API is loosely similar to Creole's, so this change shouldn't require any major re-architecture.

New features that come along with this move you might have to upgrade for include the newer DSN format for build/runtime properties, the removal of the mysqli adapter, SPL autoload integration and a few API changes you'll have to look out for. You can find out more information on the Symfony framework's support for these changes on the sfPropelPlugin and DbFinderPlugin pages.