Relink to file OR Replace contents

I found a work-around that will relink a placed AI file without relinking other instances of it AND mainting the same size and placement:

1. Place and transform the AI file

2. Convert it to Emdeded (Layer/Smart Object/Embed Linked)

3. Duplicate the embedded art layer

4. Convert it to Linked (Layer/Smart Object/Convert to Linked)

5. It will prompt you to save an unneeded PDF file (which you can ignore and delete later)
6. Now Relink it to the other AI file, and it will come in at the same size/placememnt and not affect the original instance.

7. Convert the original instance to back to Linked by right-click/Link to File and select the original file 

Simples! 

Embedded and linked smart objects have different behaviours.

For an embedded SO, one can use the Layer > Smart Objects > New Smart Object via Copy to make a copy of the SO which isn't synchronised to the others.

This command existed before Photoshop could create linked SO layers. Now that there are both embedded and linked SO layers, I believe that this command should be clarified by adding the word embedded to it: New Embedded Smart Object via Copy

The feature doesn't exist for a linked SO, as this is the whole point of linking.

I created a script to automate the creation of a "New Linked Smart Object via Copy" for a user here who wanted a linked SO to behave the same way as an embedded SO:

https://community.adobe.com/t5/photoshop-ecosystem-discussions/why-is-new-smart-object-via-copy-greyed-out-for-linked-smart-objects/td-p/14371680#U14378637

But no wat to update a single placed image which I think is an important need!!!!

The point of multiple instances of a Smart Object seems to me to be that they are instances of the same image/graphic and should therefore update together. 

Could you elaborate on your process and why you create multiple SO-instances but then need to break the connection? 

One can work around the issue by duplicating the image, relinking/replacing and then dragging the one instance back to the original image and removing the next lower Layer, which can be automated with a Script. 

Maybe even with an Action.