Which method of introducing change offers advantages like reduced risk due to the option of reverting to the old system, yet also entails higher expenses?

The method that offers the advantages of reduced risk due to the ability to revert to the old system while also involving higher expenses is parallel running. In this approach, both the old and new systems operate simultaneously for a certain period. This means that if issues or failures arise with the new system, users can rely on the old system until those problems are fixed, ensuring business continuity and reducing the risk of disruption.

Parallel running can indeed be more expensive since it requires resources to support both systems at the same time. This includes costs for hardware, software, personnel, and possibly training users to operate in both environments. The benefits, however, such as increased confidence in the new system's reliability and functionality, make parallel running a favored method for managing change, particularly in environments where operational stability is critical.

In contrast, other methods like direct changeover, pilot running, and phased implementation have different characteristics and trade-offs, which do not align with the specific advantages and expenses highlighted in the question.