I think what you are saying is you currently rent out an entire property, but going forward you just want to rent out a room.
What you can do is just add an additional listing (in the Your Listings section) for the bedroom.
Remember to block out the calendar for the entire property from the date onwards. Also, if the bedroom is in the same property you currently rent out, then remember to block out the immediate dates that are not available.
I believe that once you have completed all reservations for the 'entire property' listing you will be able to remove it, but not until then.