With the current version of MPP we can only update/overwrite existing exported pages if they have the exact same URL structure. When exporting pages to help them index better, I like to create 3 versions of page titles. Then I tick the box to "Auto-generate URL based on title". If I want to update content on these pages in the future using the "Overwrite" feature then it does not work.
It would be good if you can find a way to make this work and also it would be great if we could drip feed the process of updating/overwriting existing pages.