Most CRM Solutions work with UID (Unique identifier) for records
For exporting and/or importing data we always use the UID where possible.
Due to using the UID we ensure that we can Update imported/exported Data at anytime.
I.e. if you have a contact former name: Sue Miller, now she married and her name is Sue Frank, we can still find this contact and update some data, because of the UID as this stays unchanged.
As we are using this feature-set, we can update any data in Databases, also a few month after the conversion.<