For quick giggles and grins ...

If you load a clean Zen Cart v1.3.9f to a test directory ... then copy your Live Database to a Test Database ... and update the configure.php files from the Live site to the Test site and run the Upgrade in /zc_install ...

Set your Template to Classic on the Test site ...

and you can usually get a feel for most things such as test a Discount Coupon ... without doing much more than that ...

Granted later you have to do a real upgrade for the files to merge your customizations and check any add ons etc. but this is a quick way to test many things ...