A PhotoPost license grants you the right to run one instance (a single installation) of PhotoPost on one web server and one web site for each license purchased.
You can not have both operational on one license.
You can leave the vbGallery available as I stated but you would need to lock the Pro up via htaccess or something so it is not web accessible. Your licensing permits you to have a non web accessible test copy up for testing purposes. What I was suggesting with a test install above is that you could install Pro locked down to the public via htaccess do an import verify everything is as you wish then you can switch but once you make Pro active to the public you would need to switch off vbGallery.
The only way to do what you wish if you want both active would be to use one of your other Pro licenses you currently have if your not using it somewhere else. Otherwise you would need to purchase a new license to leave both intact.
Hope that clarifies the issue. It is pretty much the same as vbulletin you can only have one active install running on one license. Allowing users to install multiple instances while only purchasing one license would put software companies out of business.