Unfortunately, when you turn off eBay Sync, you cannot keep your existing eBay listings. eBay LINK will create new listings on eBay for you. You will lose your sales history/people watching the listings but all reviews will remain.
We recommend that you set up eBay LINK first, then end all listings on eBay via eBay Sync. The best way to do this is outlined in this FAQ: How to turn off eBay Sync and set up eBay LINK