Looking for experiences with large CRM transformations and migrations from a very old highly customized CRM solution (CRM / Sales Cloud + CX / Service Cloud) with 3k+ users to a green field new CRM implementation.   Hoping to hear from peers who have done it phased and also hoping to hear from someone who migrated knife edge to the new platform.  Would appreciate insights into why you choose that migration approach and what were the challenges you encountered?  For phased, how did you choose and prioritize the phases? Capabilities? Cohorts? What was the relative cost to the overall program in %'s for either time and/or total cost to setup the integrations necessary for this approach? What was your biggest challenge once you had users running in both legacy and on the new platform? For Knife edge, how did you test and get the users ready for all of the changes in the new platform? What was the initial adoption like and what were the main challenges?

In phased CRM transformations, prioritization was driven by critical capabilities and user cohorts, managing integration complexities. Challenges included parallel operations and data synchronization costs. For knife-edge migration, thorough user training and testing were pivotal. Initial adoption varied, with challenges centered on adapting to a radically different platform and addressing user resistance to change. Clear communication and robust support mechanisms were essential in both approaches.

Phase and knife have pros and cons so, depending on your level of preparedness, and the amount of data that needs to be migrated, I think it's up to you to choose the best approach.
Personally, I would always go for phase migration for a lot of reasons.
It may be an IT nightmare but, our data is our backbone so, so before I completely place "all of my eggs in the new basket", I would like to be absolutely sure that it delivers. I mean, my system is tried, in your case customized to your own size and needs as well, so I would never jeopardize my entire system if I did not have a failsafe.
But that's just me and I'm a bit of a paranoid geek when everything I worked so hard for is at risk.

Top-down Approach