Any project that will lead to the phasing out of one or more systems. Most replacement projects have multiple goals and will also develop new features. Examples of systems that are replaced in replacement projects range from "ancient" legacy systems to outdated content management systems on web-sites.
It is also interesting to note that there are many similarities between a project that is developing a new product (that will compete with existing products) and replacement projects. In both cases the new system is competing with one or more existing systems.