The on-disk structure (ODS) of the databases created under Firebird has changed. Although Firebird 2.0 will connect to databases having older ODS versions, most of the new features will not be available to them.

Make transportable gbak backups of your existing databases--including your old security.fdb or (even older) isc4.gdb security databases--before you uninstall the older Firebird server and set out to install Firebird 2.0. Before you proceed, restore these backups in a temporary location, using the old gbak, and verify that the backups are good.