I had this same issue. I spent hours resetting up files. I can't tell from the video, but are you hitting the 'p' hotkey? you can select your new attachment, then go to the skin you want to attach it to, find the correct slot, hit 'p,' and then select the old png. It should snap your same size images in the exact spot.
Also, one work around to having thousands of attachments in your Spine file is to make a lot of folders locally with the same PNG names. For example, I have a character that changes clothes. I have one folder name 'regular' and one folder named 'cowboy' and one spine file. In my spine file, i can call different image folders in the tree. This way I only ever have to make one spine animation file, and i have the engine calling different texture folders. It has saved me many hours of re-setting up files! And makes creating a new character as easy as hitting the spine export script.
I hope something can fix your issue, because I know how mind-numbing fixing all of those pngs can be!