For 1808 we will have the release comparison page for SAP S/4HANA. https://help.sap.com/viewer/25d2ae41d2d74803be976f9c4bda6986/SHIP/en-US
You can now check the changes of the migration objects between the releases on the web-page above.
The web-page contains the differences between 1802 to 1805 and 1805 to 1808.
The 1805-1808 comparison is available at the bottom of the comparison table.
You should also read the “Prerequisite” to know how to unhide the technical structure and field names in the migration file template. The comparison is done via the technical names.
It is also possible to download the page into a PDF file to watch it offline. The link is also available via SAP Note 2568909.