You should definitely create a new resource. Each Software Resource is an Inventory item as well as it can contain a delivery item. So you should be careful about your detection rules as if a Detection rule evaluates True Inventory will say that software exists on that PC.
So if you use the same Software Resource for two different versions of software your Inventory will get mixed up.
"Update existing resource" is for where you are packaging a piece of software that Inventory has already discovered and created a Software Resource for so you are just adding a package, programs and detection rule to it.