I believe this may be easier to do with just the wordpress default backend because you will need to understand a bit of the ID's for all your taxonomies that the National Listing posts are attached to as well as the ID's of the Parent post type.
For example I have a relationship record below in my XML file.
Note that you see the post_type = owner, this tells me that the related post type is called Owner and the Post it is related to is the post with id 350
So when you export All your National Listing it will have the data that is relative to that Post Type, this means the relationship and taxonomy data is going to be in the XML file for you to be able to modify it.
Where the domain will be the name of the taxonomy and nicename will be the term and ![CDATA[Articles]]> will be the frontend name.
So if you're changing the post from one term to another then you will need to change the nicename to the slug of your term, in my example the slug is 'articles' and the name is 'Articles' .
The find and replace will work but then you will need to know the data itself that you want to change and the information on the new data to be added.
Please let me know if this helps or if further clarification is needed.