To just get it functional I would say most of the work is in common (updating the defines, moving things in the new subfolders etc.), plus maybe some tweaks in the interface if you changed things there as well. To get it completely patched, it might take longer, since some decisions have been heavily changed (like the ones for converting from tribalism to feudalism), and there seem to be some other balance issues indirectly caused by the update. Although Birthright might be easier on that last part, since it is in an earlier stage.
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