Engineering design is a complex experience for students to undertake and for instructors to assess. Conducting tests is a critical design practice, yet the research on K-12 students' conceptions on conducting experiments is limited. Such research is essential as we attempt to understand how students become informed designers and ways in which we can support their transformation. By analyzing students' prioritization and re-prioritization of design strategies after participating in a design activity, this study examined how students' conceptions of design activities change over time. The study took place in three middle schools with 746 students. The main data source was the "conceptions of design test," which students completed a pre- and post-test, before and after completing a design project. We performed McNemar tests to quantitatively analyze students' changing conceptions of design. Results suggest that after a design activity, "conducting tests" became a statistically more important concept to students. Future work will investigate student rationale for this increased importance placed on "conducting tests" in design.