Linden Lab have announced Viewer-Managed Marketplace Released: Migration Begins July 23, 2015. This is a very important blog post for people who have a Second Life Marketplace store as the blog post explains :
Once your store is migrated, you’ll need to use the Second Life VMM Viewer to manage the inventory in your Marketplace store.
So first things first, when will the migration be taking place? Well let’s go back to the blog post for that information because the migration will not be an overnight change :
The Viewer-Managed Marketplace (VMM) Viewer is now a release candidate, and automated migrations will begin on July 23, 2015. Migrations will be done weeknights between 9:00 PM and 9:00 AM Pacific. Merchants with large stores (over around 5,000 listings) will be scheduled, and we will send an email (to the email address associated with your store) to provide you with advance notice.
For all Merchants, we will send an email at the beginning of the migration process, and another email once it has completed. Merchants will not be able to modify items in their stores during this time, but purchases will continue for any item that is not in the process of being migrated.
Therefore you will be notified before your store is migrated, but you should keep an eye on your spam folders in case emails are incorrectly flagged.