Script to Replace Contents of linked smart object, even if nested

I have a master file with multiple smart objects.
The linked smart objects only (including nested) need to be replaced/relinked to a new file via prompt.

I have 2 different image links in the same master file. 

By replacing just one of the images that link to File 1, all instances linked to the same file are updated. I do this again to replace 1 of the images linked to File 2. As I need to do this for many new files I need to automate it. 

I have tried many scripts that will successfully replace contents of the active smart object, linked or not, but none that can access the nested linked object.

I am also not sure if it the command should be 'replace contents" or "relink to file". Manually both of these commands do the job, but in the script I am not sure which is most suited.

What would be the best method to achieve this? I don't have too much experience with scripting, but I do manage to edit and customise existing scripts. Any help would be much appreciated.