I have an active D7 site with a dozen + content types and hundreds of nodes, many cross referenced to each other. If I perform an initial migration, that will give me a baseline content dump to start re-configuring and re-theming from. However I am expecting that it will take quite some time to get all the pieces working together properly. During this period the production site will have had many updates.
Is there any workflow/technique that would allow for a secondary migration/overwrite/dump/merge-purge that could deal with the changes between the old/new sites?

Thanks for any suggestions.