The rental on the base of the chair would probably not start new. It may if your patient has previously been renting a standard weight wheelchair and the new documentation shows progression/need for a lightweight wheelchair.
For example, if the patient has been renting a lightweight wheelchair, you would have to exchange the wheelchair for a reclining wheelchair.
You would only receive 6 more months on the base for a total of 13; however, you would be able to start the 13-month billing cycle for the reclining back (and elevating legrests, if not previously rented) at the time of delivery.