There are several ways to accomplish this, and yes, coonsidering that you have/will have a separate location to try it out, you want to try/test before doing so on the live database...
First of all, doing database only upgrade on the database will place the store in maintenance mode. This update on the live database really should only be done when ready to place the new version of the store online.
So, following instructions such as:
http://www.zen-cart.com/entry.php?3-...d-of-upgrading
You would want to create a new database as part of the 1.5.4 install, and import the copy of your live database and then do a zc_install database only upgrade as part of seeing what all will be involved when ready to place zc 1.5.4 online. The new database (as part of installation/generation of new configure.php files) should have the same DB_PREFIX as your live store to make all things easier to do.
As for the database backup/restore, you could use the database backup plugin on both sites to backup theold and restore the new... You could from phpmyadmin export the live database and then import the live to the backup site usng the same reverse process... There is a FAQ on "backing up your site" that applies.
http://www.zen-cart.com/content.php?144
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