Maintenance of existing software is more expensive than original development. Code goes through various changes over time. The original developers are no longer available to answer questions, or too many cooks have stirred the pot. The current collection of code cannot be modified without generating a domino effect of troubles. All of these factors cause the ongoing system to cost more for each modification.

We can create a roadmap of the currently existing program.  We can generate documentation that specifies the current state of the software. We can add comments to the existing code, to make it more maintainable in the future. In addition, we can provide guidance in making changes to the existing software that will allow for more cost-effective modifications in the future (see Refactoring).