It is not tricky, but systematic. You can import records several times. Each import must match the Customer ID or what ever primary key for that table
the matching information.
You can select what import fields match your modified DB.
One import Customer ID and Contact Info in one table. Import again for Customer ID and Invoice data... etc.
I think you are right. I am testing it out now and when it comes to the time to do the actual switch I shall do it all again...dont think it is worth writing a script for..