New online store design experience is currently in beta.
At this time the transition process isn't clear though there are enough substantial differences that theme in use will need to be upgraded.
I've made a thread on this and other online store features
Thanks for your question :) The new Online Store Design Experience represents the biggest change to how the Online Store Editor and themes work, and will introduce a few new types of sections. There will be "content sections", like slideshows, galleries, etc that you will be able to add to any page in the same way you can add regular sections to the home page now. These content sections will be stored within a /content directory of your theme. There are also sections for pages, which are described in our developer documentation.
When the feature is available, existing themes will need to have the new folders added, or you can install an updated version of your theme, to start adding these new sections. As mentioned during Reunite, the new feature is available to partners participating in the developer preview beta, so that they are familiar with how this feature works. In the coming months we'll be providing docs, videos, and guides to ensure the process for implementing the new Online Store Design Experience is as straightforward as possible.
We also have a dedicated community forum board where you can find more info on this feature, if you have any other specific questions, this is the best place to ask.
Hope this helps Janislanka :)
Shopify | Developer Community Manager