For some customers, maintenance is the last item that requires professional services. But building a software solution isn't usually like building a stone obelisk and placing it in the middle of your piazza. For most people, it's more like planting a garden, and will require not only tending but will evolve over time based on which plants thrive.
The future may bring changes that require modifications to the solution. Government agencies must ensure that their policies and procedures are consistent with laws and regulations which are constantly changing. This regulatory compliance is not optional.
What if you come up with an improved business process and want to reflect it in your CRM solution? Such changes could affect the data model, automated workflows and reports.
In order to control the cost and disruptions of change, you may want to set up a release schedule to set expectations of users who request enhancements. Over the course of an implementation you may move to less frequent releases.
Don't forget that changes require notifications to users and sometimes refresher training. A mechanism to solicit user feedback can identify ways to improve and boost user satisfaction at the same time.