Are there examples of successful data migration? Are there known limitations of migrating data from old systems with over 20 years of historical information? 

Barhead has done many projects requiring data migration from old legacy applications to Dynamics.  

For simple migration (and ongoing integration with 3rd party Fundraising platforms), Fundraising and Engagement provides Donation Import to upload, validate and create or update supporters and their respective donation records.  

For more complex scenarios involving interactions such as phone call records, notes, emails and large volume of records, we follow an ETL (Extract Transform Load) approach and use SSIS (SQL Server Integration Services) for the migration. Moreover, we only store recent transactions in Dynamics, such as 5-year donations, and the rest in the Azure SQL database. We leverage Power BI to provide transaction reports, dashboards and insights.