Adapting and integrating after difficult acquisitions

What are some best practices for helping employees of small companies acquired under ‘difficult’ (not necessarily hostile) situations quickly adapt to the new company culture, strategy, etc.? For example, where smaller companies that are founder-led and with familial cultures are pulled into large corporate environments. The goal is to retain key talent of the acquired company, but a big corporate culture and environment isn’t everyone’s cup of tea. What’s worked for others: assigning mentors to drive engagement, allowing flexibility to preserve some of the ‘old’ cultures, etc.?