There is no functionality in the product to do this. I do not believe the way files are stored in the product just a phpmyadmin query would do it either. In Photopost Pro a query could do it but vbgallery the files are stored via userid under the files folder not by category id.
I suppose if you change the folder id's to the proper new user and do some trickery in phpmyadmin it can be done but not something easy to say the least.