I know this post is a few months old now, but I was just searching for this exact problem and found no one seems to know how to do it. After a little trial and error I finally worked it out.
To do this it’s best to make a master anim BP first that all others will derive from. Once you have this, right click on it and select “Create Child Anim Blueprint with Skeleton” and select the original skeleton as it’s probably the only one in the list, but we will change that next.
Now open the new child AnimBP and click on Class Settings. In the details panel under Advanced you can find Target Skeleton which can now be changed to any skeleton in the project.
Once that has been changed, compile and save and the preview character should update to a broken version of itself because it’s using the old animations for the wrong skeleton. If this doesn’t update, close the window and reopen it.
You can now use the Asset Override editor, which is a panel next to details. If it’s missing, open it from Window->Asset Override Editor.
In the override editor, you can now go through the animGraph and find all the animation states and change them to animations that use the new skeleton.
From here, any changes you make to the master anim BP will also update all your children anim BP’s so you can easily make changes to all of them.