Mergers and acquisitions (M&A) are often being initiated with the promise of synergy, scalability and a high chance for competitive advantages. Yet many M&A integrations fail to realise their intended value in practice (Koi-Akrofi et al., Weber & Tarba, 2012). This suggests that the underlying causes more complex than the structural considerations and decisions and require a closer look at how change is experienced and implemented within everyday work reality. This research explores ‘how formal organizational change efforts interact with informal practices during post-merger integration (PMI), and what this interaction implies for achieving integration goals and anticipated synergies’. Central in this research is the role of ‘bootlegging’: the unofficial, often invisible practices employees develop to continue operations when formal structures are experienced as too slow, rigid, or incomplete.This research is based on an in-depth qualitative case study of a recently merged company. The case company was created by bringing together two legacy entities with different histories, structures, and ways of working. To understand how the integration unfolded in practice, fifteen semi-structured interviews were conducted with employees and managers from different hierarchical levels of both entities. Existing literature on change management, M&A, and bootlegging was used to develop an initial framework of organizational, social, human, and cultural factors that influence integrations and change implementations. In contrast to many top-down models, this research uses a bottom-up perspective in which bootlegging and other informal practices are treated as a central analytical dimension, allowing the framework to reflect how change is actually being experienced in reality.The findings show, that during this period of change, employees experienced a large leadership distance, fragmented communication, and increasing operational complexity. Together, these struggles contributed to four reinforcing gaps: a credibility gap between communicated intentions and perceived reality; a synergy gap between promised and realized value; an execution gap between decisions and implementation capacity; and a visibility gap between operational efforts and what higher levels actually acknowledge. In response to unclear expectations, resource constraints, and cultural frictions, employees developed informal workarounds and trust-based information lines to bridge the operational challenges. These bootlegging practices were rational, adaptive responses that allowed operations to continue, but limited knowledge sharing, learning and the realization of M&A synergies.Based on these insights, the framework was iterated to represent a clear bottom-up, human-centered perspective on change experiences and readiness within PMI. It introduces dimensions, such as leadership visibility, system compatibility, psychological safety and voice, and vertical connectivity as key conditions for ‘building a change-ready organisation’. This framework was used as a (design) tool in co-creation sessions with employees, translating analytical insights into concrete needs, priorities, and directions for action. The outcomes were defined as strategic design implications, being the base for a strategic plan. The strategic roadmap is supported by a tactical roadmap that specifies stakeholder roles, ways of working and the expected outcomes over time.This research contributes by explicitly positioning bootlegging as a central mechanism in M&A PMI, presenting how informal practices simultaneously compensate for and reinforce the challenges and shortcomings of formal change efforts. It offers a practical approach for decision-makers to understand and work with these dynamics, moving from a top-down focus to a realistic view on how change is actually experienced and acted upon in post-merger environments.