Various initiatives for undergraduates from historically underrepresented backgrounds attempt to address disparities in the completion of science, technology, engineering, mathematics (STEM) degrees and the pursuit of careers in scientific research. Intensive research training programs for historically underrepresented undergraduates may include multiple components, such as authentic research experiences, advising and mentoring, supplemental curriculum, and financial assistance. Following comprehensive support during program participation, the post-program transition may present a vulnerable period in students' career trajectories. This study used a community-based participatory research (CBPR) approach to investigate the experiences of students completing an intensive research training program to understand and develop recommendations for the post-program transition process. As a team of program alumni, academic researchers, and program staff, we developed, conducted, and analyzed semi-structured, open-ended interviews of recent program alumni and students approaching program completion (n=11; 55% female, 55% non-White). Applying thematic analysis at semantic and latent levels through a critical paradigm revealed the transition as a bittersweet experience, with feelings of pride and accomplishment mixed with sadness and anxiety. Findings also suggested the transition is described as a narrative influenced by preceding program experiences and adaptations. Financial concerns were prominent, and specific barriers and facilitators of successful transition included: aligned mentoring, negotiation of continued research employment, consideration of culture, planning for next steps, and engagement with the scholar community. Collaboratively, we developed recommendations for program improvements potentially relevant to similarly intensive STEM diversity programs. We also highlight the value of a CBPR approach that includes students equitably as co-researchers in program research and evaluation.