Migrating of Data from Non-Validated System to Validated System

I am working on validating a new computer system and the business wants to migrate the data from legacy system which is non-validated into this new validated system.

As part of the data migration, we will be doing extensive migration test. However, my question is, since the data is coming from a non-validated system, is there anything else we should do?

I am thinking, Data Risk Assessment prior to migration to ensure data integrity should help, any suggestions will be appreciated.

What kind of system is it?