@Sharyn51 This is a way to try to get around the rules that say Airbnb's cannot be transferred to a new owner. It will fail however in that co-hosts do not have full access privileges to the account.
Another way around the rules would be to add yourselves as co-hosts and then get the existing owner to change the payout details to your account, the email to your email and then continue to operate under their name. Even this is likely to go wrong if you ever have to prove identity (because it won't be your name on the account.
If I were you I would just set up a new listing basically copying all the current text and make this go live at completion. Can you trust the existing host to move money to you as you work through the current bookings post completion? If not then it is a real mess and the current host will have to cancel existing bookings and they may or may not book with you.