You can't use pre-2.0 security database in firebird 2.0 or higher directly.
  If you try to put old security.fdb into firebird's new home directory with 
new (security2.fdb) name, you will get a message - cannot attach to password 
database. That's OK and is by design. In order to be able to use old database, 
you must run appropriate upgrade script - security_database.sql. To do so:
  1. Put your old security database in some known to you place (not new home
directory). Always have a copy of it!
  2. Start firebird using it's new, native security2.fdb.
  3. Connect to your old security database as SYSDBA and run the script.
  4. Stop firebird.
  5. Copy upgraded database to firebird's home directory (as security2.fdb).
  6. Set LegacyHash parameter in firebird.conf to 1.
  7. Start firebird.

  Now you should be able to connect to firebird 2.0 server using your old 
logins and passwords. But be careful - as long as you have LegacyHash = 1,
firebird's security doesn't work completely. You must change SYSDBA password,
let users change their passwords (yes, since 2.0 each user can change his 
password himself) and set LegacyHash back to default value of 0.