In this demo we show how the Lorien operating system supports lightweight, efficient and safe online changes to any aspect of the software running on sensor nodes - and how this promotes reuse of deployed sensor networks through run-time software evolution. Lorien is based on three principles: i. pure dynamic component design enabling lightweight software evolution to all parts of node software ii. abstract architecture description promoting independence and persistence of software evolutions iii. system integrity rules promoting safety of online changes to any part of node software We believe that Lorien is unique in its support for safe, incremental online software evolution, and in the scope of this support which ranges from the lowest level drivers through protocols and application components. This goes significantly beyond contemporary WSN operating systems such as TinyOS, Contiki and SOS which either offer only offline image-based software updates or else provide only limited-scope online software evolution that is restricted to application-level code and lacks strong integrity support.