from my understanding, there are or coule be changes to the database once your core is up and running, hot fixes, service packs, etc, can change the schema of your DB and if you were to try to install a fresh build to the DB, it would / could wreak havoc on the existing db.
It is really simpe and quick to create a new DB instance, build your new core to it, then cut over to your existing one.
If you had installed SP2 on the old core / db, install it on the new one BEFORE cutting over.
Also, it might be advisable to install any recent component patches (hot fixes) that have not been superseded before moving to the existing DB
You can choose a existing LDMS 9 database during your new core server installation. Just choose "New Database" during the setup process and point the database configuration to the existing LDMS 9 database. Setup process will know this is a proper LDMS 9 database and will create the new core server preserving the existing database information. The new core should have the same name as the previous core, otherwise you must change the coreserver name in the metasystem table.