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.
There are a number of different ways to integrate with eBay and eBay Sync uses a different method to to create listings that is not compatible so your existing eBay listings are not able to be kept.
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