Thank you for your interest in our product.
My name is Denis, I am a CS-Cart sales manager. I will be glad to answer your questions.
Please, let me explain: there is no ability to upgrade from CS-Cart to Multi-Vendor; these products are independent and work separately.
If you would like to have Multi-Vendor software, it is necessary to purchase a separate Multi-Vendor license (https://www.cs-cart.com/multi-vendor-yearly-subscription.html). The price of a Multi-Vendor yearly subscription is USD 590.
Please also note that you can use the Import tool (CS-Cart Documentation — CS-Cart 4.16.x documentation) to preserve and import some of your data from the existing CS-Cart installation.
I hope this information will be useful to you.
Feel free to ask further questions if any.
I will be glad to help.
If you are interested in the upgrade or want to buy Multi-Vendor license at the lowest possible price, please contact us. We will be happy to assist you.