We're a 28-person consultancy, and run into this a lot. One approach that we've found to work is to help the client bring on at least one in-house employee (contract or full-time) with enough technical expertise to conduct updates and improvements. We then shift into a "leadership" role, maintaining overall responsibility for the product's continued success, but guiding most of the low-leverage day-to-day work to their in-house guys. Management, by necessity, is much more amenable to "task-switching" than is hands-on technical work, and represents a harder-to-find skillset for many clients, so it's typically not too difficult to sell this kind of continued engagement.