They are different but have the same concept of product development. In some way, we can consider Item selling on Themeforest is the Basic Theme and on Cmsmart is Advances solutions.  On Themeforest, most of the clients are web developer, freelancer and private start-up who just need a basic theme to quickstart the website.  On Cmsmart, we prefer to offer our Advances solutions to clients who want to have more features and VIP support.
It depends on your decisions to buy which products you want and as they are different we can not upgrade from the item on Themeforest to Cmsmart. The only way is to have a new installation, not upgrade. We can consider giving a discount for a buyer who bought Items on Themeforest and now wants to switch completely to solutions packages on Cmsmart, but not a refund.