You need to create your properties in Rentals United if you haven't done so already.
Select Airbnb in "ADD CHANNELS" and go through the checklist. When you click "connect" we will create your properties in Airbnb in a new account (new property IDs).
Now you just need to inform by email to email@example.com that you have begun an integration from your old "manual account" to your new "automatic account". Provide your old IDs and your new IDs to them.
This transfer process would automatically "unlist" the old manually-created properties -- although any reservations associated with those properties would remain live -- and transfer the reviews over to the new properties.