When exporting photos, that already exists in that location, it would be great to have one additional option: "Overwrite only modified photos", so that I do not have to manually select the ones that I adjusted.
You can accomplish this by "Publishing" to hard drive instead of Exporting. Go to Publishing Services and click the + sign to add a new service. Choose harddrive. You'll likely have to create collections (smart or otherwise) and add your photos to them and then publish the collection(s) it will only overwrite photos which have been modified.
There may also be some 3rd party plugins available to export changed photos. But this is kinda already native to the publishing feature already present in LR.