After a utility finances efficiency upgrades at a site, what happens to the obligation for cost recovery when a new customer signs up for service at that meter?

The successor customer (whether renter or owner) will enjoy the net savings from the investment, and until cost recovery is complete, the customer will likewise be obligated to pay the fixed, on-bill charge, which is significantly less than the estimated annual savings. Due to the on-bill payments made by the prior customer(s) served by that meter, a successor customer will have fewer billing cycles remaining until cost recovery is complete, at which point they will enjoy 100% of the savings.