As this was a very simple & limited protection, user/password are now only used by sync (see File > Database Properties) before importing databases on other devices. Encrypted databases are much more secure and recommended now.
You could either create a new encrypted database and move everything from the old database into the new one. Or you could create an encrypted sparse image (see /Applications/Utilities/Disk Utility.app), copy the *.dtBase2 database to the volume, unmount the volume and change the extension to .dtSparse.
As sparse images require only the necessary disk space, it’s recommended to use a generous maximum size.