No, when you sign into the new reference manager your existing library will sync automatically.
One of the most exciting features of the new reference manager is the "Real Time Sync". Rather than having to click "sync" to sync your library between Mendeley Desktop and Mendeley Web this will happen automatically whenever you add new documents or make a change. In the case of import, as soon as you add a new document, you will notice that the sync icon will change to "Syncing" and then go to a "Green" to indicate that "sync" has been completed. This means that the new documents have also been uploaded to your other versions of Mendeley.