Convert a rental to a purchase

In our business we often rent out product before we sell it.  We have implemented the rental processing in NS yet, but I recently came across a white paper on the Skyytek web site that proposed a possible pipeline of using a service item for the rental tracking and locations for rental stage tracking (IE: Out for rental, available for rental)

I like this solution, it makes sense and will be easy to implement.

My next question is how to convert a rental into a purchase.
For example, a customer is renting a device and we have billed them for the rental, they then call and say they would like to purchase it.  Now what?  The only way to handle this I can think of is to create a new Sales Order and supply the SN of the rental for the SN of the purchase.