How do I handle in-flight sales deals during a CRM AI assistant migration?

We're migrating from an old CRM to a new one that has a built-in AI assistant for deal notes and forecasting. The cutover plan is clear for closed deals and new ones, but I'm stuck on the active pipeline.

Specifically, how do you migrate the context of in-flight negotiations? The old system has hundreds of email threads, call logs, and custom field history attached to those open opportunities. The new AI tool needs that context to be useful from day one.

Is it better to do a full historical data dump, or just migrate a "deal snapshot" summary? How have others handled the training period where the team is learning the new AI while trying to close existing deals?